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Id-139: Disease Management Disease Management Paul Vincelli Diseases of crops can occur whenever Crop rotation is thus one of the most able, disease resistance should be the a disease-causing agent is in contact important disease control practices for foundation for economical disease with a susceptible host plant in an envi- corn production worldwide. control. ronment that is favorable for disease de- Pathogens that are not as effec- No single corn hybrid is resistant velopment. These three fundamental tively controlled by crop rotation in- to all diseases present in Kentucky. ingredients are necessary for a disease clude those that do not survive in the Furthermore, the importance and to develop and are often referred to production field itself. For example, prevalence of corn diseases vary from as the disease triangle (Figure 1). rust fungi that attack corn overwin- one farm to the next and from one Understanding this fundamental ter south of Kentucky and are blown year to the next. These facts can com- relationship helps us understand dis- into our corn fields each season on plicate the hybrid selection process. ease management since all disease wind currents. Pathogens that attack Nevertheless, an informed decision management practices presented in a wide range of field crops are also less can be made by selecting hybrids with this publication affect one or more effectively controlled through rota- resistance to the diseases most likely sides of the disease triangle. For ex- tion. For example, the charcoal rot to be a problem. Resistance to other ample, planting a hybrid with some pathogen can attack corn, soybean, diseases should be considered on a sec- resistance to gray leaf spot targets the and grain sorghum and is not con- ondary basis. host side of the disease triangle. Crop trolled through a corn/soybean rota- When selecting a hybrid, the pro- rotation helps to starve a pathogen (dis- tion. Likewise, pathogens that persist ducer should recognize that there are ease-causing agent) by depriving it of indefinitely in agricultural soils are different levels of disease resistance. its food source; this affects the patho- not effectively controlled through If available, agronomically acceptable gen side of the disease triangle. Delay- crop rotation. An example is the fun- hybrids with high levels of resistance ing planting until soil temperatures gus Pythium ultimum, the most com- usually provide the best protection exceed 50°F reduces the amount of mon cause of damping off of corn. against a serious disease outbreak. seedling damping off by targeting the Hybrids may also exhibit moderate or environment side of the disease triangle. Resistant Hybrids even low levels of resistance to par- ticular diseases. This means that, One of the most practical and eco- while the disease still can develop on Preplant Decisions nomical means of disease control is to these hybrids, lower incidence of dis- That Affect Disease select agronomically suitable hybrids ease can be expected in most circum- with adequate resistance to diseases Development stances than on a fully susceptible of concern on your farm. Unfortu- Most of the agronomic decisions hybrid. For some diseases, low to mod- nately, resistance is not available for corn producers make have some im- erate resistance is all that is available some diseases. However, when avail- pact on disease development. In fact, once a corn field is planted, a producer’s disease management pro- Figure 1. Disease triangle. Disease only develops when three conditions are met: a pathogen gram is essentially in place, for better infects a susceptible host under disease-favorable conditions. or worse. Thus, consider your preplant decisions as disease-management de- cisions also. favorable environment Crop Rotation favorable environment Many corn pathogens survive be- tween crops in the corn residue, and some do not attack other field crops commonly grown in rotation with susceptible pathogen corn. Consequently, rotating to wheat, host (cause) susceptible pathogen soybean, or other crops helps to starve host (cause) certain corn pathogens that survive in the field by depriving them of a food source as the crop residue decomposes. NO DISEASE DISEASE 33 among current commercial hybrids, ter during spring under conservation Foliar sprays of fungicides may be even though higher levels of resis- tillage. economical in seed corn fields to pro- tance would be desirable. In these While conservation tillage systems tect against a variety of leaf diseases. cases, use of a hybrid with even a low may favor certain diseases, they can They may also occasionally be justi- level of resistance is usually superior also reduce pressure from certain oth- fied for production of certain specialty to planting a susceptible hybrid. ers. For example, charcoal rot, which corns. However, fungicide sprays typi- Sometimes a moderate level of resis- is favored by high soil temperatures cally do not show justifiable economic tance is acceptable for fields where re- and low soil moisture early in the returns for commercial production of duced disease pressure is expected. growing season, would be expected to dent corn. However, under high disease pressure, be worse in a conventional system low to moderate levels of disease re- than a no-till system. Scouting for Diseases sistance will not provide adequate dis- The possibility of enhancing pres- While it is not possible to know ease control. Such hybrids may sure from certain diseases under con- with complete certainty which dis- require you to pay greater attention servation tillage is not necessarily an eases will develop in a given season, to other disease management strate- argument to return to conventional the disease history of the farm and gies in order to achieve good results. tillage. However, producers should area will indicate the diseases most Hybrids can also be selected for recognize situations when their pro- likely to occur. A disease history for a tolerance—the ability to yield well duction system may enhance disease farm is established by scouting fields even though symptoms develop. In- activity so they can employ other dis- and identifying disease outbreaks formation on disease-tolerant hybrids ease management practices in order when they occur. Your county Exten- is limited, but tolerant hybrids can be to maintain adequate levels of disease sion agent, farm supply dealer, and useful when available. control. crop consultant can also be good It is important to plant more than sources of information. However, one corn hybrid on your farm. Plant- Other Cultural Practices farm-specific information obtained ing one hybrid is like “putting all your Other preplant decisions can also through field scouting is the most re- eggs in one basket.” Should a disease influence disease activity. For ex- liable basis for developing a farm dis- problem develop on that hybrid, your ample, early planting tends to en- ease history. Unless you are absolutely whole crop is at risk. Planting several hance activity of Pythium seedling certain as to the cause of a particular hybrids helps to spread the risk of losses diseases, which are favored by cool, problem, have the condition diag- from disease. wet soils. Conversely, late planting nosed by a reputable field specialist, can enhance pressure from gray leaf or submit the sample to the Univer- Tillage spot, a late-season disease that is more sity of Kentucky Plant Diagnostic Lab. Conservation tillage systems pro- damaging on younger crops than more vide for less soil erosion, less fuel con- mature ones. Mycotoxins sumption, savings of time and labor, Plant populations are usually se- Several mycotoxins—toxins pro- moisture conservation, and easier lected on the basis of hybrid charac- duced by fungi—can occasionally be double-cropping. Conservation tillage teristics and yield potential of the field. found in shelled corn. Aflatoxins oc- systems can, however, increase pres- Some diseases can be more severe at cur very infrequently in Kentucky, but sure from certain diseases, especially high plant populations; several of the when they occur, they are often asso- under continuous corn production. stalk rot diseases are examples. A fer- ciated with hot, dry weather during Prime examples are gray leaf spot and tility program that is inadequate or grain fill or with improper storage con- Diplodia ear rot. Spore levels of the excessive, or in which major nutrients ditions. Fumonisins can also sometimes fungi that cause these diseases are are not in proper balance, may also be found in Kentucky corn, as can higher in fields where previously in- enhance disease activity. vomitoxin (also called deoxynivalenol, fected corn residue is left on the soil or DON), and zearalenone. More in- surface. When residue is tilled into the Fungicides formation on mycotoxins in corn can soil, spores are trapped underground Essentially all corn seed is treated be found in the Extension publications and cannot easily spread to before purchase with fungicides to help Aflatoxins in Corn (ID-59) and Myc- aboveground plant parts. Further- control seed rots and seedling diseases. otoxins in Corn Produced by Fusarium more, buried crop residues decompose This provides inexpensive protection Fungi (ID-121). faster, which reduces pathogen sur- against stand loss, should conditions vival. Activity of seedling diseases can favor these diseases after planting. Un- also be increased in no-till systems treated seed should be treated with fun- because soils remain cooler and wet- gicides before planting.
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