CROP PROTECTION – THE DISEASESCorn generally suffers little from fungal diseases. This "ecological advantage" can be explained by the origin of the plant and the fact that corn was bred in the past without the use of pesticides. The risk of an infestation is reduced by numerous preventive measures. The implementation of strategies adapted to the level of risk limited.
Adapting practices for disease restriction
In order to limit as much as possible the risk of disease infestation, several rules must be followed:
1. Regular monitoring of plots to identify at-risk varieties or areas. 2. Managing crop residues to reduce the potential for infection or harborage of disease vectors such as borers, along with sufficiently early harvest dates and tillage (plowing) adapted to the level of risk, etc. 3. An appropriate selection of varieties in the affected regions at the appropriate time: Recommendation for Helminthosporium- or Fusarium-tolerant varieties. Under dry continental conditions in summer. Corn in rotations has a lower risk of certain diseases. However, depending on the year and area, corn can suffer damage in places. Good variety selection and efficient integrated pest management allow control of helminthosporiasis and fusarium, which pose the highest risk. Leaf diseases can result in high losses if they are necrotizing diseases, infestation occurs very early and harvest occurs late.
The most common diseases
The root necrosis
Due to the development of various interrelated fungi (phytiume, fusarium, rhizoctone), the root necrosis at the time of harvest is noticeable by the fall of numerous corn plants.
To limit the development of an infestation after identifying at-risk areas or varieties, it is important to follow best agronomic practices: preparing and plowing the soil under good conditions and reworking a sufficiently recompacted soil.
In addition to these precautions, targeted treatment of seed furrows, z. B. with azoxystrobin from the chemical family of strobilurins, a good solution approach, especially against root necrosis.
There are different variants of helminthosporiasis with different degrees of harmfulness. Infestation begins after flowering and can be devastating, causing a breakdown in the photosynthetic activities of the corn. In severe, early infestations with rapid development, yield losses can range from 20 to 50% of the kernels.
Knowledge of the risk situations is a prerequisite for the implementation of appropriate preventive measures.
The Risk is to the plot highest when the following conditions are combined:
– an area where endemic helminthosporiosis has developed or a heavy infestation was detected in the previous year; – repeated use of susceptible varieties, especially in monocultures; – surface soil preparation (no-till), no crushing of crop residues; – dry sheds or drying facilities nearby.
The Control of helminthosporiosis therefore requires simple agronomic preventive measures:
– monitoring of plots; – fine shredding and burying of corn residues; – selection of less susceptible varieties.
Blight diseases: corn boll blight and corn head blight
The Corn boll blight is most widespread. The spores of Ustilago maydis are naturally present in the environment of the field crop and are very volatile. Infestation often starts from "natural" (silks) or entry sites caused by tie lesions, z. B. by pest infestation (corn flies), the phytotoxicity of herbicides, the rejection of kernels or cobs due to drought stress. Fungus develops in the young organs that are still growing (leaf tips, buds, nodes, inflorescences, etc.).). This disease is rarely harmful. There are no solutions for treatment. The control of the risk factors listed above is the most effective method, but without any guarantees. There is some direct or indirect varietal sensitivity (sensitivity to pests, injury).
The Corn head blight is a very rare disease that can cause significant yield losses. The typical symptoms of corn head blight caused by Sphacelotheca reiliena are only visible after flowering in the reproductive organs, the panicles and cobs. Inocula are found mainly in the soil. The development of the disease in the plant are favored by all factors that slow down the growth of the young plant, z. B. early dry periods and compacted soil. Therefore, the disease often occurs on the edges of the plots. Certain varieties are particularly susceptible.
Other leaf diseases: Rust, eyespot disease, anthracnose
Serious adverse effects of these diseases are rare, and the economic impact is low. Undermining of crop residues, plowing, and crop rotations limit the risk of infection. In contrast, there is a possibility that monocultures and no-till may increase the risk that the diseases will develop.
Rust and cabbage blight are more common in areas with an oceanic, fresh climate and after flowering. Anthracnose occurs at an earlier stage (after flowering). In warmer climates with more frequent rainfall on.
Symptoms may vary among cultivars, but the low incidence of the disease and its moderate harmfulness in hybrids have not yet justified specific cultivars.
Advice on preventive measures
The interactions between climate, the technical process and the sensitivities of the different varieties require the simultaneous implementation of different technical approaches to avoid cumulative effects:
– Residue management and tillage immediately after harvesting the previous crop: surface residues are a potential source of infection for the following corn, both for Fusarium and borers. Efficient shredding of stalks after harvest and early, surface incorporation of residues in the form of mulch following a stubble field shredding operation promote decomposition of organic material that can be a carrier of pathogens. The shredding of the residues and the uprooting of the roots also have the advantage of preventively destroying the overwintering opportunities of the borer and corn rootworm. However, recommendations depend on crop conditions and climate. However, they are easier to implement if harvesting does not take place too late in the year. – Early sowing dates combined with early maturing corn varieties allow to avoid the risk of Fusarium infestation of cobs due to earlier maturity, as these diseases occur mainly in mild and wet autumns. Early sowing results in lower moisture content in the grains by bringing the cycle forward, when higher humidity in the fall favors the spread of fungi. – Variety selection: The choice of an early-maturing variety adapted to the area and an early harvest date remains the most important precautionary measure. When seeding without prior plowing and tillage for effective residue decomposition, avoid more susceptible varieties. The information provided by the grower, or otherwise the observation of the plots on the part of the farmer during the 4 weeks before harvest, allows the most sensitive plants to be removed; – The protection of the plots against borers and corn rootworms (entry holes for fungi) is essential when the risk of infection justifies it. The strategy here is to limit populations, especially those of caterpillars of the 2. Generation. harvesting dates that take into account the state of health of the grains. If symptoms are detected, do not harvest too late after ripening to contain the development of fusarium and the accumulation of toxins. Management also involves assessing the risks a few weeks before the expected harvest date, taking into account the crop associated with cultivation, the weather conditions of the year, the diagnosis of Fusarium infestation and, finally, the weather forecast for the coming weeks.
Corn and mycotoxins: a controlled risk
The implementation of the European regulation (Nr. 1887/2006) for cereals with regard to mycotoxin risks obliges increased attention to the causes that trigger their occurrence.
If there are toxins, there must also be pathogenic fungi that produce them. This condition is necessary, but not sufficient. The presence of Fusarium on the cob does not necessarily induce toxin production. Extensive studies have been conducted in Europe to classify and prioritize the reasons for the occurrence of this risk. It very soon became apparent that climatic conditions were a predominant cause of infection and development of Fusarium on a cob and production of toxins: Rainfall at the time of female flowering and in the following days, humidity and the temperature after flowering and at the beginning of autumn. The stress caused by drought and certain temperatures, the presence of borers, be exposed to pathogens.
For DON and zearalenone secreted by Fusarium graminearum, harvest date and maturity stage, cultivar susceptibility, and management of residues from previous field crops are risk factors. Fumonisins are secreted by Fusarium (mainly Fusarim verticillioides and Fusarium proliferatum), which are rather saprophytic fungi. So it is the injuries caused by boring insects (borer, root borer, heliothis) on the cobs and the stem, or cracks caused by drought stress, or also the second infestation after the infestation of already existing pathogens, which favor the development of fumonisins. Just as with Fusarium graminearum, there is some varietal susceptibility to these fungi.
Feed manufacturers fear especially the presence of aflatoxins (B1 or B2). These are attacked by certain fungi (z. B. Aspergillus flavus), which multiply mainly on grains that are kept under moist and warm storage conditions. The risk, which is lower in temperate climates, is higher for corn in southern and eastern Europe.