By: R.W. Heiniger, Crop Science Extension Specialist, NCSU
There are three key diseases that are often controlled by either by fungicides or management practices that can have significant impacts on organically grown corn. Growers should be aware of these diseases and select hybrids and management practices that reduce the risk these diseases pose. While there are many other diseases that can attack corn, it is rare for them to cause economic loss. Pictures of these field corn diseases can be found on the web at: http://www.btny.purdue.edu/Extension/Pathology/CropDiseases/Corn/
Seed Rots and Seedling Blights (caused by species of Fusarium, Stenocarpella, Pythium, and other fungi). These diseases are often associated with the term "damping-off”. Plants die either at emergence or within a few days of emergence. These diseases are more prevalent in poorly drained, excessively compacted, or cold, wet soils. Planting old or poor quality seed with mechanical injury to the pericarp will increase seed rot and seedling blight, as will planting seed too deep in wet, heavy soils. Seed vigor ratings are often used to select hybrids with genetic resistance to seed rots and seedling blight and this technique can be used with organic production.
There are a number of fungicides approved for use in certified organic production systems that can be used for control of diseases — mainly non-synthetic compounds or biocontrols. These include: neem (and its derivatives), hydrogen peroxide, potassium and sodium bicarbonate, pesticidal soaps and copper products. While these products do have potential for controlling some diseases in corn, no research has been done on them in field corn in North Carolina and, therefore, we can give no recommendations for their use in the state. The cost of these organically approved pesticides may be prohibitively expensive for field crop production. Conditions for use of a pesticide must be documented in the organic system plan (NOP 2000).See the Pest Management Web Resources for more information, or check out the OMRI site.
Stalk Rots (caused principally by the fungi Stenocarpella zeae and species of Fusarium as well as Colletotrichum graminicola). Stalk rots are present each year and may cause considerable damage, particularly if abundant rainfall occurs during the latter part of the growing season. Stalks previously injured by cold, leaf diseases, or insects are especially susceptible to attack by these fungi. Diseased stalks ripen prematurely and are subject to excessive stalk breaking. Stalk rots not only add to the cost of harvesting but also bring the ears in contact with the ground, increasing their chance of rotting. Adequate fertility (particularly adequate potassium) is the key to controlling stalk rot.
Charcoal Rot (caused by the fungus Macrophomina phaseolina). Charcoal rot becomes most evident with the onset of hot dry weather. It may cause stalk rot, stunting, and death of the corn plant. This disease is often considered to be stress related. Typically when this disease occurs in North Carolina soil fertility and pH are at very low levels. Although the fungus typically survives in the soil, rotation is not generally an option since most crops are susceptible to this disease. Therefore, reduction of nutrient and water stress is the principle means of control. Hybrid resistance has not been documented.