PERTUMBUHAN TANAMAN JAGUNG

Secara umum jagung mempunyai pola pertumbuhan yang sama, namun interval waktu antartahap pertumbuhan dan jumlah daun yang berkembang dapat berbeda. Pertumbuhan jagung dapat dikelompokkan ke dalam tiga tahap yaitu (1) fase perkecambahan, saat proses imbibisi air yang ditandai dengan pembengkakan biji sampai dengan sebelum munculnya daun pertama; (2) fase pertumbuhan vegetatif, yaitu fase mulai munculnya daun pertama yang terbuka sempurna sampai tasseling dan sebelum keluarnya bunga betina (silking), fase ini diidentifiksi dengan jumlah daun yang terbentuk; dan (3) fase reproduktif, yaitu fase pertumbuhan setelah silking sampai masak fisiologis.
Perkecambahan benih jagung terjadi ketika radikula muncul dari kulit biji. Benih jagung akan berkecambah jika kadar air benih pada saat di dalam tanah meningkat >30% (McWilliams et al. 1999). Proses perkecambahan benih jagung, mula-mula benih menyerap air melalui proses imbibisi dan benih membengkak yang diikuti oleh kenaikan aktivitas enzim dan respirasi yang tinggi. Perubahan awal sebagian besar adalah katabolisme pati, lemak, dan protein yang tersimpan dihidrolisis menjadi zat-zat yang mobil, gula, asam-asam lemak, dan asam amino yang dapat diangkut ke bagian embrio yang tumbuh aktif. Pada awal perkecambahan, koleoriza memanjang menembus pericarp, kemudian radikel menembus koleoriza. Setelah radikelmuncul, kemudian empat akar seminal lateral juga muncul. Pada waktu yang sama atau sesaat kemudian plumule tertutupi oleh koleoptil. Koleoptil terdorong ke atas oleh pemanjangan mesokotil, yang mendorong koleoptil ke permukaan tanah. Mesokotil berperan penting dalam pemunculan kecambah ke atas tanah. Ketika ujung koleoptil muncul ke luar permukaan tanah, pemanjangan mesokotil terhenti dan plumul muncul dari koleoptil dan menembus permukaan tanah.

Benih jagung umumnya ditanam pada kedalaman 5-8 cm. Bila kelembaban tepat, pemunculan kecambah seragam dalam 4-5 hari setelah tanam. Semakin dalam lubang tanam semakin lama pemunculan kecambah ke atas permukaan tanah. Pada kondisi lingkungan yang lembab, tahap pemunculan berlangsung 4-5 hari setelah tanam, namun pada kondisi yang dingin atau kering, pemunculan tanaman dapat berlangsung hingga dua minggu setelah tanam atau lebih.
Keseragaman perkecambahan sangat penting untuk mendapatkan hasil yang tinggi. Perkecambahan tidak seragam jika daya tumbuh benih rendah. Tanaman yang terlambat tumbuh akan ternaungi dan gulma lebih bersaing dengan tanaman, akibatnya tanaman yang terlambat tumbuh tidak normal dan tongkolnya relatif lebih kecil dibanding tanaman yang tumbuh lebih awal dan seragam.
Setelah perkecambahan, pertumbuhan jagung melewati beberapa fase berikut:
Fase V3-V5 (jumlah daun yang terbuka sempurna 3-5)
Fase ini berlangsung pada saat tanaman berumur antara 10-18 hari setelah berkecambah. Pada fase ini akar seminal sudah mulai berhenti tumbuh, akar nodul sudah mulai aktif, dan titik tumbuh di bawah permukaan tanah. Suhu tanah sangat mempengaruhi titik tumbuh. Suhu rendah akan memperlambat keluar daun, meningkatkan jumlah daun, dan menunda terbentuknya bunga jantan (McWilliams et al. 1999).
Fase V6-V10 (jumlah daun terbuka sempurna 6-10)
Fase ini berlangsung pada saat tanaman berumur antara 18 -35 hari setelah berkecambah. Titik tumbuh sudah di atas permukaan tanah, perkembangan akar dan penyebarannya di tanah sangat cepat, dan pemanjangan batang meningkat dengan cepat. Pada fase ini bakal bunga jantan (tassel) dan perkembangan tongkol dimulai (Lee 2007). Tanaman mulai menyerap hara dalam jumlah yang lebih banyak, karena itu pemupukan pada fase ini diperlukan untuk mencukupi kebutuhan hara bagi tanaman (McWilliams et al. 1999).

Fase V11- Vn (jumlah daun terbuka sempurna 11 sampai daun terakhir 15-18)
Fase ini berlangsung pada saat tanaman berumur antara 33-50 hari setelah berkecambah. Tanaman tumbuh dengan cepat dan akumulasi bahan kering meningkat dengan cepat pula. Kebutuhan hara dan air relatif sangat tinggi untuk mendukung laju pertumbuhan tanaman. Tanaman sangat sensitif terhadap cekaman kekeringan dan kekurangan hara. Pada fase ini, kekeringan dan kekurangan hara sangat berpengaruh terhadap pertumbuhan dan perkembangan tongkol, dan bahkan akan menurunkan jumlah biji dalam satu tongkol karena mengecilnya tongkol, yang akibatnya menurunkan hasil (McWilliams et al. 1999, Lee 2007). Kekeringan pada fase ini juga akan memperlambat munculnya bunga betina (silking).
Fase Tasseling / VT (berbunga jantan)
Fase tasseling biasanya berkisar antara 45-52 hari, ditandai oleh adanya cabang terakhir dari bunga jantan sebelum kemunculan bunga betina (silk/rambut tongkol). Tahap VT dimulai 2-3 hari sebelum rambut tongkol muncul, di mana pada periode ini tinggi tanaman hampir mencapai maksimum dan mulai menyebarkan serbuk sari (pollen). Pada fase ini dihasilkan biomas maksimum dari bagian vegetatif tanaman, yaitu sekitar 50% dari total bobot kering tanaman, penyerapan N, P, dan K oleh tanaman masing-masing 60-70%, 50%, dan 80-90%.
Fase R1 (silking)
Tahap silking diawali oleh munculnya rambut dari dalam tongkol yang terbungkus kelobot, biasanya mulai 2-3 hari setelah tasseling. Penyerbukan (polinasi) terjadi ketika serbuk sari yang dilepas oleh bunga jantan jatuh menyentuh permukaan rambut tongkol yang masih segar. Serbuk sari tersebut membutuhkan waktu sekitar 24 jam untuk mencapai sel telur (ovule), di mana pembuahan (fertilization) akan berlangsung membentuk bakal biji. Rambut tongkol muncul dan siap diserbuki selama 2-3 hari. Rambut tongkol tumbuh memanjang 2,5-3,8 cm/hari dan akan terus memanjang hingga diserbuki. Bakal biji hasil pembuahan tumbuh dalam suatu struktur tongkol dengan dilindungi oleh tiga bagian penting biji, yaitu glume, lemma, dan palea, serta memiliki warna putih pada bagian luar biji. Bagian dalam biji berwarna bening dan mengandung sangat sedikit cairan. Pada tahap ini, apabila biji dibelah dengan menggunakan silet, belum terlihat
struktur embrio di dalamnya. Serapan N dan P sangat cepat, dan K hampir komplit (Lee 2007).
Fase R2 (blister)
Fase R2 muncul sekitar 10-14 hari seletelah silking, rambut tongkol sudah kering dan berwarna gelap. Ukuran tongkol, kelobot, dan janggel hampir sempurna, biji sudah mulai nampak dan berwarna putih melepuh, pati mulai diakumulasi ke endosperm, kadar air biji sekitar 85%, dan akan menurun terus sampai panen.
Fase R3 (masak susu)
Fase ini terbentuk 18 -22 hari setelah silking. Pengisian biji semula dalam bentuk cairan bening, berubah seperti susu. Akumulasi pati pada setiap biji sangat cepat, warna biji sudah mulai terlihat (bergantung pada warna biji setiap varietas), dan bagian sel pada endosperm sudah terbentuk lengkap. Kekeringan pada fase R1-R3 menurunkan ukuran dan jumlah biji yang terbentuk. Kadar air biji dapat mencapai 80%.
Fase R4 (dough)
Fase R4 mulai terjadi 24-28 hari setelah silking. Bagian dalam biji seperti pasta (belum mengeras). Separuh dari akumulasi bahan kering biji sudah terbentuk, dan kadar air biji menurun menjadi sekitar 70%. Cekaman kekeringan pada fase ini berpengaruh terhadap bobot biji.
Fase R5 (pengerasan biji)
Fase R5 akan terbentuk 35-42 hari setelah silking. Seluruh biji sudah terbentuk sempurna, embrio sudah masak, dan akumulasi bahan kering biji akan segera terhenti. Kadar air biji 55%.
Fase R6 (masak fisiologis)
Tanaman jagung memasuki tahap masak fisiologis 55-65 hari setelah silking. Pada tahap ini, biji-biji pada tongkol telah mencapai bobot kering maksimum. Lapisan pati yang keras pada biji telah berkembang dengan sempurna dan telah terbentuk pula lapisan absisi berwarna coklat atau kehitaman. Pembentukan lapisan hitam (black layer) berlangsung secara bertahap, dimulai dari biji pada bagian pangkal tongkol menuju ke bagian ujung tongkol. Pada varietas hibrida, tanaman yang mempunyai sifat tetap hijau (stay-green) yang tinggi, kelobot dan daun bagian atas masih berwarna hijau meskipun telah memasuki tahap masak fisiologis. Pada tahap ini kadar air biji berkisar 30-35% dengan total bobot kering dan penyerapan NPK oleh tanaman mencapai masing-masing 100%.

Sumber: Balai Penelitian Tanaman Serealia, Maros (Nuning Argo Subekti, Syafruddin, Roy Efendi, dan Sri Sunarti).

Effectively Managing Northern Corn Leaf Blight

Sweet corn growers in New York, and elsewhere in the Northeastern USA, now need to include managing northern corn leaf blight (NCLB) in their production program. 2012 was the first year NCLB occurred at a notable level in some areas, including on Long Island. Growers were caught by surprise. Marketability of ears was affected when symptoms developed on husks because it gave them an old appearance, and the quality of the ear was affected. NCLB is expected to occur in 2013 because this disease was common in 2012 and the fungal pathogen can survive over winter in infested crop debris. Additionally it produces spores easily dispersed by wind.
NCLB is not a new disease of corn (it was first reported in NY in 1878), but it had not been observed in some areas for many years. On Long Island it had been at least 20 years. It had been reported before 2012 as increasing in importance on field corn in the northeast and on sweet corn in New England. Increase in disease occurrence likely reflects change in the pathogen such that it is no longer suppressed by resistance genes in field corn varieties. Race 0 is thought to still dominant, but Race 1 has been detected; it overcomes the main major resistance gene, Ht1. Greater disease development in field corn results in more inoculum to affect sweet corn. Another factor may be storms that have been occurring during August with patterns that facility pathogen movement and disease development. NCLB and southern corn leaf blight are also known as Helminthosporium leaf blight, which is important to know because the later name is used on some fungicide labels.
The pathogen produces an abundance of spores that are dispersed by wind. Numerous spores were observed on spots examined microscopically. This likely is how the pathogen moved to areas like Long Island recently, and why the disease was widespread in 2012. A biologically similar pathogen, which causes southern corn leaf blight, moved from the Gulf of Mexico to Canada during one season in the 1970s.
Favorable conditions for the pathogen are moderate temperatures (64 - 81 F) and leaf wetness from rain, dew or fog for at least 6 hours. Conditions during August 2012 evidently were very favorable as that is when symptoms of NCLB were observed commonly in sweet corn plantings on Long Island.

Symptoms of NCLB and rust

Leaf spots are moderately large and long (1 to 5 inches), elliptical, and grayish green becoming tan with age. Their shape resembles a cigar or boat.  Similar to rust, this disease can impact ear quality when it develops on husks. Left unmanaged, NCLB can develop rapidly causing a crop to become completely blighted and appearing as affected by frost.
A primary reason to manage this disease is to minimize symptoms on husks, which can cause them to look old. Additionally, yield can be reduced when NCLB is severe. Grain yield of processed corn reportedly can sustain losses of up to 50% when the disease is established before silking; minimal losses in yield occur when the disease is delayed until 6 weeks after silking.

The dusty look to this NCLB lesion is the pathogen spores.

Cultural management practices include incorporating debris after harvest and rotating crop land. However, since the pathogen produces spores easily dispersed by wind, these practices may not contribute as much to control of this pathogen as for others that produce larger, heavier spores. The benefits to soil health of reduced tillage in many cases will outweigh the benefits of reducing initial inoculum by plowing in debris. Growing a resistant or less susceptible variety is an effective practice. ex0876 7143 is resistant.  Obsession was less severely affected by NCLB than Beyond and ACR7196 in a variety evaluation conducted in FL. More varieties exhibit some resistance in the large variety evaluations conducted each year in IL (see Table). Providence appears to be among the more severely affected varieties based on observations from commercial crops on Long Island in 2012.
To determine when to apply fungicides for NCLB, each week inspect crops thoroughly for symptoms, focusing on older leaves, and check for updates on occurrence of NCLB in local extension newsletters. The potential for NCLB to develop will increase with successive crops. Applying a protectant fungicide (e.g. Bravo or Dithane) might be worthwhile when NCLB has been reported in the area but symptoms are not found in the planting. Using a spray boom with drop nozzles will increase spray coverage on leaves low in the canopy, which is important because NCLB begins to develop there.
There are 2 groups of targeted fungicides effective for NCLB: FRAC Group 11 (which includes Quadris and Headline) and Group 3 (Bumper, Fitness, Proline, Propimax, AmTide Propiconazole, and Tilt). Corn is on a supplemental label for Proline. Targeted fungicides are more rain-fast than protectants and have a longer period of activity. Alternate among these groups and tank mix with a protectant fungicide to manage resistance developing in the pathogen. Starting early in disease development, when very few symptoms are present, is critical to successful control of most fungal diseases, including NCLB. The maximum number of applications that can be made to a crop is 6 for FRAC Group 11 fungicides, with no more than 2 sequential applications, and 2 to 4, depending on the product, for Group 3 fungicides. Other fungicides labeled for NCLB contain active ingredients in both FRAC fungicide groups: Headline AMP, Quilt and Stratego YLD.  These are a good choice when only one or two applications are economical, as is often the case with field corn. The pre-harvest interval (PHI) is 0 days for Proline and Stratego YLD, which both contain prothioconazole. Stratego YLD also contains trifloxystrobin. PHI is 7 days for products with the other FRAC Group 11 fungicides. It is 14 days for fungicides with propiconazole as an active ingredient, which includes most of the FRAC Group 3 fungicides currently registered and the combination product Quilt. All these fungicides have a 12-hr REI.
Several fungicide evaluations have been conducted recently in Florida where NCLB has been an important disease in sweet corn for several years. Other foliar diseases also occur there. The protectant fungicides tested exhibited similar efficacy. Control was enhanced by using a spreader sticker. They were not as effective as the targeted fungicides. The FRAC Group 3 fungicides (aka triazoles) were more effective than the FRAC Group 11 fungicides (strobilurins) for NCLB; the opposite was the case for rust. FRAC Group 11 fungicides were very good for controlling NCLB at high label rates. Among the combination products, Headline AMP was the most effective and Stratego was the least, being similar in efficacy to fungicides with only a FRAC Group 11 active ingredient.
To effectively manage both NCLB and rust, growers should alternate among FRAC Group 3 and 11 fungicides, adjusting the program based on which disease is most important. Stratego is a good choice when an application is needed near harvest because the PHI is 0 days while it is 7 or 14 days for the other fungicides. Apply all targeted fungicides with a broad-spectrum fungicide for resistance management.

source : Effectively Managing Northern Corn Leaf Blight in 2013
Margaret Tuttle McGrath
Department of Plant Pathology and Plant-Microbe Biology, Cornell University
Long Island Horticultural Research and Extension Center; 3059 Sound Avenue
Riverhead, NY 11901; mtm3@cornell.edu

Maize dwarf mosaic potyvirus

Virus description — MDMV is a member of the potyvirus group, which represents the main vegetable-infecting group of viruses. The virus is transmitted in a nonpersistent (stylet-borne) manner by several aphid species. The virus is acquired in less than 60 sec., transmitted in less than 60 sec., but is not retained by the aphid for any length of time (usually less than 1 hr.). An aphid must revisit an already infected corn plant or a perennial grass reservoir to reacquire the virus.
MDMV has not been a factor for corn production in the state for 15 or more years. Natural hosts have included field and sweet corn, grain sorghum, Johnsongrass (Fig. 4) (which is not important in New York), and various perennial weeds like deer’s-tongue (Panicum clandestinum) (Fig. 5), commonly found in the Hudson Valley. Interestingly, cultivated wheat, barley, oat and rye are not susceptible. Five strains of MDMV are recognized, with strain A considered as the type strain. MDMV strain B is now classified as a strain of sugarcane mosaic virus. MDMV is seed-transmitted in dent corn at very low frequencies (0.007 to 0.4%).

Figures 4 and 5. Hosts of maize dwarf mosaic virus include Johnsongrass(Sorghum halepense) Left and deer's-tongue (Panicum clandestinum)(Right). Photos cortesy of T. A. Zitter

Disease symptoms — Systemic mosaic symptoms can appear on most varieties of sweet corn that do not carry tolerance derived from dent corn (Fig. 6). Like BYDV, the earlier the plant is infected, the more plant damage can occur. This consists of plant stunting, failure to set ears, and blanking of tip ends (Fig. 7).

Figures 6 and 7. Early whorl leaves showing symptoms of maize dwarf mosaic virus (Left). Poor ear fill is also common on plants that were infected at the time of corn pollination (Right). Photos courtesy of T. A. Zitter

Aphid vectors — At least 15 species can transmit MDMV, including the greenbug (Schizaphis graminum), the English grain aphid (Sitobion [Macrosiphum] avenae), the oat bird cherry aphid (Rhopalosiphum padi), and the corn leaf aphid (Rhopalosiphum maidis). Many factors can influence the frequency of transmission by aphids.
Management guidelines — MDMV occurred in at least three areas of the state last season (Hudson Valley, Capital District, and western New York), but we can only speculate on reasons for its occurrence in 2000. Movement of viruliferous aphids into corn has been previously noted, and with the heavy occurrence of common rust this last year, air movement of rust and aphids is possible. The chance for seed transmission is very remote, and would not explain the occurrence over such a wide area, with such a wide divergence in variety selections. Although sweet corn varieties change frequently, we are unaware if some varieties are more susceptible than those grown in past seasons.
The management of MDMV is very difficult because of the nonpersistent manner of spread. Insecticides can reduce aphid populations, but do not prevent virus transmission, and at best would only slow the rate of virus spread.

source :

Date 5-2001

COOPERATIVE EXTENSION • NEW YORK STATE • CORNELL UNIVERSITY

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by Thomas A. Zitter, Department of Plant Pathology,
Cornell University, Ithaca, New York 14853

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Virus Diseases

Only two virus diseases are currently recognized as infecting sweet corn in New York. They are maize dwarf mosaic virus (MDMV) and maize white line mosaic virus (MWLMV). MDMV has been present in the state since the late 1960s; MWLMV was first recognized in the United States in Tompkins County, New York, in 1979. These two diseases differ greatly in their method of transmission and their locations within the state.
Maize dwarf mosaic virus (MDMV) was first reported in the state in 1967 and, at that time, was a serious problem for both field and sweet corn. Today, most field corn hybrids are resistant or tolerant to MDMV, and the genes responsible for resistance in field corn have been transferred into sweet corn progenies. Because dent corn is the source of resistance for MDMV, the release of high-quality sweet corn varieties has been delayed. In addition to infecting all corn types, MDMV also infects many grass species (annual, winter annual, and perennial). Although many strains of MDMV have been identified, the two most common strains are called A and B. MDMV-A can infect perennial johnsongrass (Sorghum halepense), which is its principal weed source, whereas MDMV-B and the other strains cannot infect johnsongrass. However, other grass species can serve as virus reservoirs for strain B.
Both strains of MDMV are transmitted by several aphid species in a nonpersistent manner, with the corn leaf aphid (Rhopalosiphum maidis) being the predominant vector. MDMV remains an important virus disease of sweet corn in key fresh-market producing areas of the state, such as the Hudson Valley and selected upstate locations, and for processing sweet corn in several upstate counties.
MDMV symptoms on sweet corn foliage consist of a mosaic pattern of light and darker green streaks along the veinal and interveinal tissues (fig. 1). Symptoms usually persist, but are most striking at the whorl stages. Mosaic symptoms are also evident on the flag and husk leaves (fig. 2). Inside, the ears will show poor cob and tip fill (fig. 3). Infected susceptible plants are stunted and have reduced yields when compared with resistant plants inoculated at the same time (fig. 4).
 Several sources of MDMV inoculum are recognized, but opinions differ as to the importance of these sources. MDMV may be seedborne in corn, but this occurrence is rare and the percentage of infestation is low. A second source of virus is the movement of viruliferous aphids (virus ready for transmission) for long distances on low-level winds often associated with storm fronts. This source was substantiated in several midwestern states and, therefore, cannot be ruled out. A third virus source is perennial and winter annual grasses that can maintain the virus from one season until the next; the aphids acquire the virus from these sources and transmit it to the susceptible corn crop. This source would appear to be the most plausible and important, but proof is difficult.
Several methods for controlling MDMV are available. Although MDMV-resistant sweet corn varieties are being developed, they currently lack good eating quality, are not suitable for processors, and are not presently available in all maturity groups or colors to span the entire planting season. Previous work in the Hudson Valley indicated that in most years growers who planted susceptible varieties after June 15 ran the risk of reduced yields from virus infection for corn harvested in September. Thus, planting early to avoid peak aphid flights should be done wherever it is practical. Herbicides should be used to eliminate johnsongrass and other perennial grass hosts bordering fields. Although insecticides are effective in controlling aphid populations, they cannot prevent the introduction of virus by migrant aphids. Mineral oil sprays have been used successfully in other states to control viruses, including MDMV, transmitted nonpersistently by aphids. However, this technique depends upon special spraying apparatus plus thorough and timely application of oils to a fast growing crop and may not be practical for most growers.
Maize white line mosaic virus (MWLMV) is a relatively new virus disease of corn, first identified in the United States at Ithaca, Tompkins County, New York. Since then the virus has been reported from seven additional states located in New England and the north central region (Ohio, Michigan, and Wisconsin). Since 1979, the virus has been found in 21 counties in New York, where it is more extensively distributed than in any of the other states.
MWLMV is not transmitted mechanically or by aphids or leafhoppers. It is, however, soilborne and appears to be transmitted by a soilborne fungus. Several fungi known to parasitize corn roots act as vectors for plant viruses; the fungi require ample soil moisture to allow zoospores to move in the root zone and make contact with roots of germinating seedlings.
Symptoms of MWLMV in corn seedlings appear approximately 1 month after planting into infested soil. A strong mosaic pattern with short chlorotic white lines 1/ 16 inch wide and up to 3/4 inch long appear in and along the veinal tissue (fig. 5). Some particularly susceptible sweet corn varieties display "goose-necking" of the entire plant (fig. 6). Severely infected plants will fail to produce ears, whereas ears on other plants are poorly developed and unmarketable. The disease is usually associated with lower, wetter areas of fields, but as fungus/virus-infested soil and debris are redistributed, infected plants may appear in level areas of the field. Not all plants express symptoms even though virus can be recovered from roots and stems. This suggests that the virus may be more extensively distributed than is now recognized.
Because the virus is soilborne and depends upon soil moisture for infection, avoid planting in low, wet areas. Select varieties that are less susceptible; contact your Cooperative Extension agent for an up-to-date list. Because the virus can infect all corn types (sweet, dent, Indian) as well as grasses, infested fields should be rotated out of corn for several years.

Smut of Sweet Corn

The smut of corn (Ustilago maydis) was probably present when white people first came to America. It is now present in nearly all countries where corn is grown and is of great economic importance in North America. Sweet corn is more susceptible than field corn and, under very favorable conditions, may become infected during the seedling stage.
The plant may be infected at any time in the early stages of its development, but gradually grows less susceptible after the formation of the ear. Any part of the plant above the ground can be invaded, although it is more common on the ears, the tassels, and the nodes than it is on the leaves, the internodes, and aerial roots. The boil is composed of a white, smooth covering, enclosing a great mass, sometimes 4 or 5 inches in diameter, of black, greasy, or powdery spores. After the spores mature, the covering becomes dry and brittle, breaks open, and permits the black powdery contents to fall out.(Fig. 1)
The smut spores are blown long distances by the wind and are particularly prevalent when there is much dust in the air. They will germinate in rain water, but germinate more readily in the drainings from barnyard manure. Consequently, spores are scattered over the farm with manure and have been known to pass through the digestive tracts of animals without losing germinating ability. The germ tube of the spore ordinarily does not enter the plant directly, but a few drops of dew caught in the leaf sheath will remain long enough for the fungus to start a luxuriant growth. It is only when it is growing in this manner that it can enter the plant.
Hot dry seasons are favorable for the growth of the fungus. When the soil is dry, dust can blow more readily; and it is by means of air-floating dust that the fungus spores are carried from one farm to another. Furthermore, during drought, the usually high temperature is especially favorable for the germination of the spores. The spores, however, must have water collected in the silk, leaf blades, and other parts of the corn to permit the required amount of growth for penetrating the tissue.
How to Control
Seed treatment is of no value. Recommended control measures are rather unsatisfactory. If every gardener or corn grower in a given community would go through the field two or three times during the season, cutting out all the smut balls before they have time to break open and destroying them by burial or fire, smut could be reduced. One year of cutting is not enough to cause a noticeable difference in the amount of the disease, but if this cutting is continued for 2 or more years, the smut will gradually be reduced. This is true, however, only if adjoining neighbors also cooperate. The removal of the smut should always be accompanied by rather long crop rotations.
Although there is some degree of tolerance of smut among some of the newer varieties, in favorable hot dry summers the disease may be found in all varieties. The following have shown some tolerance: Apache, Aztec, Comanche, Sweet Sue, Bellringer, Golden Security, Merit, Calumet, Capitan, Golden Gleam, Wintergreen, Midway, Pacer, Bravo, and Gold Cup.

source : 
Fact Sheet Page 727.20 Date 1-1979
COOPERATIVE EXTENSION • NEW YORK STATE • CORNELL UNIVERSITY

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by Arden Sherff, Department of Plant Pathology, Cornell University, Ithaca ,NY

Common Rust of Sweet Corn

Common rust on sweet corn is caused by the fungus Puccinia sorghi. Epidemics of this disease can cause serious losses in yield and quality of sweet corn. High rust susceptibility of many popular sweet corn hybrids is a major factor contributing to rust epidemics. Another factor is that sweet corn is usually planted over an extended period from May through June for fresh and processing uses. The staggered planting schedules result in high concentrations of fungal spores in the air, originating from early planted fields, at the time when late-planted fields contain young actively growing susceptible plants.

Symptoms and Signs

Common rust on sweet corn appears in the field as oval to elongate cinnamon brown pustules scattered over upper and lower surfaces of the leaves (fig. 1). The pustules rupture and expose dusty red spores (urediniospores, fig. 2), which are spread by wind and have the ability to infect other corn leaves directly. As the pustules mature, they turn brownish black and release the dark-brown overwintering spores (teliospores). In severe epidemics, pustules may also appear on the ears and tassels, and the leaves may yellow and become easily tattered in strong winds. Partial resistance is expressed as chlorotic or necrotic hypersensitive flecks with little or no sporulation (compare fig. 3, susceptible, and fig. 4, resistant).

Disease Cycle

The complete life cycle of P. sorghi includes five different spore types and two hosts, corn (c) and species of wood sorrel (Oxalis spp.) ( fig. 5). The spore types and the hosts they infect are teliospores (o), basidiospores (o), pycniospores (o), aeciospores (c), and urediniospores (c). All spore types occur in Mexico, but those involving the alternate host, Oxalis spp., are of little importance in the life cycle of the fungus as it occurs in temperate areas of the United States. The aecial stage (fig. 6, called "cluster-cups") appears on the underneath surface of Oxalis leaves, producing aeciospores, which are windborne and infect corn leaves. These infections give rise to urediniospores, which are the most-important spore type in the northern United States. Urediniospores occur on corn leaves throughout the growing season and continue cyclic infections. The disease cycle for common rust is illustrated in figure 7.

Severe rust epidemics on dent corn have been rare because of the availability of resistant varieties. However, rust epidemics on sweet corn have been severe, though somewhat sporadic. Three major factors interact to influence the outbreak of rust epidemics on sweet corn: (1) the quantity of urediniospores available to initiate rust epidemics, (2) environmental factors, and (3) the level of rust susceptibility in the sweet corn varieties in use. Urediniospores are unable to overwinter successfully in northern climates. Each spring urediniospores move north from the southwestern United States and Mexico, following the sequential plantings of corn from the south up to Canada. Temperatures of 60° to 75° F (16-24° C) and heavy dews or high relative humidity (close to 100%) favor rust development. The current weather conditions influence spore germination and the rate at which rust epidemics develop. Moisture is required for spore germination. Infection will occur when leaves are wet for a minimum of 3 to 6 hours.

How to Control :

Cultural. Although most of the current popular sweet corn hybrids are susceptible to rust, resistant varieties are becoming available. Two types of resistance are being used by commercial sweet corn breeders: race-specific resistance and partial rust resistance. A partial list of hybrid reaction to rust severity at harvest, from most to least resistant, includes aRRestor, Excellency, and Prevailer (possess specific resistance with 0% rust severity and fungus unable to sporulate); Sweetie, Miracle, Country Gentleman, Sucro, Sugar Time (partial resistance); Dandy, Gold Dust, Golden Glade, Patriot, Tendertreat EH, Sugar Loaf (moderate resistance); Seneca Horizon, Gold Cup, Seneca Sentry, Kandy Corn EH, Jubilee, Sweet Sal, Commander, Stylepak, Merit, Silver Queen, Florida Staysweet, and Sweet Sue (least resistance). Resistant or moderately resistant varieties should be used for late plantings when fungal spore density in the air is likely to be high as a result of infections of earlier-planted sweet corn. The varieties listed are examples only, and no endorsement is implied. For suggestions on varieties adapted to New York growing conditions, see Cornell Recommendations for Commercial Vegetable Production.

Fungicides. Modest control of rust on sweet corn can be achieved with applications of fungicides. Trials conducted in western New York have shown that three applications of mancozeb applied by air significantly reduced disease severity on all the leaves of sweet corn plants. Fungicide applications also significantly increased the number of harvestable ears and the weight of the harvested ears. Secondary ears tended to be more severely affected by rust than primary ears. Research conducted in other states has shown that, by controlling rust with fungicides, improvements in moisture content sugar content, and ear-tip fill were observed. Timing of the first fungicide application is critical because it needs to be applied early enough to reduce the rate of epidemic development. Because rust spores arrive from outside the immediate area planted to corn, it is difficult to predict when this spray should be applied. From research conducted in western New York, a 6-pustule-per-leaf action threshold has been proposed for initiation of fungicide sprays on later plantings of susceptible sweet corn. This threshold is only a guideline and is still in the process of being validated.

Refer to the most recent issue of Cornell Vegetable Recommends for registered products for use on sweet corn and follow label directions.

Disease cycle of sweet corn rust caused by the fungus Puccinia sorghi 

DO YOU KNOW SWEET CORN PEST ?

Soil insects, primarily wireworms and white grubs, can cause stand reduction or stunted plants. These insects should be considered a serious threat when corn will be planted in ground immediately following sod. A preplant treatment may be considered. However, it is likely that a planting-time treatment will provide sufficient protection.

Wireworms

 Wireworms are on the label of most soil insecticides. These insects can be very numerous in scattered spots in a field. In these cases, damage may occur even when a soil insecticide is used.

Soil insect problems generally decrease with time out of sod. Problems with white grubs may occur in soils fertilized heavily with compost or manure. Rootworms may cause damage in ground where corn is grown every year. Soil insecticides will generally greatly reduce troubles with these pests.

Soil insecticides that are banded over the row should be incorporated lightly in the top one-half to one inch of soil. These products are very toxic and should be used with great care.

Cutworms

Cutworms are unpredictable. While they can be very destructive, the chances for damage in any given year are relatively low.

Infestations usually develop on early season weed growth. Late planting, low-damp areas of the field that drain poorly, fall or spring weed growth and the amount of service residue influence the potential for cutworm infestations. Early land preparation and good weed control will help to reduce cutworm problems. It is important to watch closely for cut plants. Early detection means an insecticide application can be made before serious damage occurs. Sprays of Ambush, Pounce, or Sevin or the use of Sevin bait should result in good control.

Flea Beetles

Flea beetles are small, black, hard-bodied insects that hop or fly quickly when disturbed. They overwinter as adults and become active early in the spring.

Flea beetles attack young corn plants as soon as the first true leaf appears. Flea beetles produce small feeding streaks or "window pane" scarring on the leaves. During wet, cold periods in the spring when corn is growing slowly, damage from this pest can be severe. Only rarely will this actually kill plants.

The real concern from flea beetles is Stewart's wilt, a bacterial disease of corn. The pathogen is carried inside the flea beetle. Young plants become infected as the beetles feed. Damage from Stewart's wilt is far more severe than leaf injury caused by the beetles. Wilt resistant sweet corn varieties should be selected to prevent losses. Chemical control of the beetle should not be the only protection program for Stewart's wilt.

European Corn Borer

Corn borer populations fluctuate from year to year and can be more severe in some fields than others. There are two generations of this insect each year.

The first generation occurs from early June to early July and is most damaging to early-planted corn. Damage is primarily due to borer tunneling in leaf midribs and the stalk. The second generation in August and September is a greater threat to late-planted corn. Borers of this generation tunnel in ears, ear shanks and stalks. Stalk breakage may be serious. Borer entrance holes in corn plants also provide a site for stalk rot pathogens to enter the plant.

The first generation is most vulnerable to chemical control. Treatment should be considered if "shot-hole" damage is apparent on 25% of the plants and live larvae are present in the whorls. One application should be sufficient against the first generation. Your county agent for agriculture can give you accurate information on when to expect damage. Treatment applied after borers have entered the plant will not be effective.

The second generation of European corn borers presents a much greater control challenge. First generation larvae were generally concentrated in the whorl, which provided a good collection funnel for the insecticide spray or granules. On tasseled corn, the second generation borers are dispersed over the plant and protected behind leaf sheaths and in axils. In this situation, plant coverage with the foliar spray is very important. Also egglaying for the second generation occurs over a long period of time. If borer populations are high, it is possible that two or more sprays may be necessary to achieve satisfactory control.

Inspect plants carefully during August. Consider an insecticide application if live borers are found on 25% of the plants.

Fall Armyworm

Fall armyworms are unable to survive Kentucky winters. Moths migrate northward from the Gulf area each year and generally arrive in Kentucky about mid-July.

Infestations are most likely to occur on corn that is knee to waist high in July. Late planted sweet corn, especially in the southern tiers of counties should be watched closely for fall armyworm activity. The larvae or worms feed on leaves and in the whorl. They will enter the ear and cause damage similar to that from the corn earworm.

Fall armyworms are smooth and green to black with three thin yellow lines down the back. A dark stripe and a wavy yellow stripe runs along each side. Larvae have a dark head with a white inverted "Y".

Eggs are laid in groups of infested plants may be found over a field. Moths and worms remain active until frost. Repeated spray application may be necessary if fields become reinfested. Thorough spray coverage with insecticides recommended against corn borers should provide satisfactory control. Applications should be made before worms enter the ear.

Corn Earworm

The corn earworm is the most serious sweet corn pest because it feeds directly on the market product. Once worms have become established within the ear, control is impossible.

Earworms spend a relatively short period of their life feeding in a site that can receive an adequate insecticide application. Earworms are variable in color, but they have a brown head without markings and numerous microscopic spines covering their body. A preventive program, especially on late season corn, is necessary to ensure that damaged ears are at a minimum.

Corn earworms overwinter as pupae in underground cells. Some adults from these pupae begin to emerge as early as late March, others may not appear until August. There are generally four generations each year, however, overlap is great and adult moths that can lay eggs may be present in significant numbers throughout most of the growing season.

Female moths search out green silks on which to lay single eggs. Following hatch, the small larvae often eat the egg shell before beginning to feed on the silk. Corn earworms generally complete their development in 14 to 16 days. Full grown worms leave the ear and pupate in the soil. The new adult will be active in another 10 to 14 days.

A preventive program against corn earworms may begin when 10% of the ears are silked. Repeated sprays at three to five day intervals until 90% of the silks have wilted should give a high percentage or worm-free ears during early and midseason. Control is more difficult late in the season. Even shortening spray intervals may produce only 90% clean ears. Spray solution should be driven deep into the silks. Corn hybrids having a long, tight-fitting shuck appear to suffer less damage than those with loose shucks.

Sap Beetles

Sap beetles, also known as picnic beetles, are primarily scavenging insects that feed on overripe or damaged fruits and vegetables and other decaying matter. Usually they are attracted to ear tips damaged as a result of corn earworm feeding. However, the beetles may enter undamaged ears anytime from early silk to maturity.

Sap beetles are about 3/16 to 1/4 inch long and brown to black. There may be orange markings on the wing covers. Larvae may be found in some ear tips along with adults.

These pests are difficult to control because even if large numbers are killed, more are likely to come in from untreated areas. Since these insects are scavengers, anything that can be done to prevent or eliminate overripe, insect- or disease-damaged fruits and vegetables will be helpful. Insecticides used to control corn earworms should be a relatively good job of reducing sap beetle problems.

Silk Clipping Insects

Rootworm beetles and Japanese beetles (pictured left) feed on corn silks. When numerous, their feeding activity can keep silks clipped very short. The results may be reduced pollination and kernel set. Sprays directed at silks to control earworms should reduce silk clipping damage also.

Typically, it requires at least two Japanese beetles or five corn rootworm beetles feeding on the silks before maiximum pollen shed before any pollination interference occurs. Sprays for corn earworm usually provide adequate control of silk clipping insects.

Corn Leaf Aphids

Corn leaf aphids are a common sight in corn in the commonwealth. These are pear-shaped soft-bodied insects. They vary from blue-green to gray and have piercing sucking mouthparts. Feeding by colonies of aphids can cause leaf discoloration, stunting, or wilting. They occur in the curl of leaves, whorl, or unemerged tassels. Aphids secrete a sugary substance known as "honeydew" which promotes the growth of black mold. Heaving infestations usually are limited to late-planted corn. Although these aphids are fairly common, generally no control action is necessary.