This article will provide a brief review of corn seed and seedling diseases and the fungicidal seed treatments (including one new product) that are used to manage these diseases.
Corn planting is off to a good start in Illinois, with about 47% in the ground on April 27 versus 24% last year on this date (estimates from the National Agricultural Statistics Service, USDA). Much of the state has good conditions for corn germination and seedling growth, while some areas have wet conditions. Where conditions are relatively warm and dry, seed and seedling diseases will have minimal impact, and seed/plant loss may not exceed the 5 to 10% loss that we may normally expect at this stage of crop development. However, conditions can become favorable for disease if they turn to prolonged cool and/or wet conditions, as we saw in May 2002.
How can seed and seedling diseases affect the corn crop? We can probably generalize and say that seed and seedling diseases may have a minor impact on corn in many fields during an average year in Illinois. Partly, this is due to widespread use of corn fungicidal seed treatments along with average conditions that don't favor seed and seedling diseases. But when soil is cool and wet after planting and emergence and growth is "delayed," these diseases can be a problem. Their main effects may be reduced plant populations and stunting. In addition, it is possible that nonlethal infection at the seedling stage may cause damage through the growing season. Not as much data as is desirable exists to document potential damage from seed and seedling pathogens, but results from two seed treatment studies provide some indication of damage caused by soilborne corn fungal pathogens. Studies from Iowa (G. Munkvold, 1998) suggested Captan+ Allegiance and Maxim+ ApronXL can provide a yield increase of 10 to 15 bushels per acre compared to nontreated seed, and results from Illinois (W. Pedersen, 2000) were similar with these same products (10- to 15-bushel-per-acre increase over nontreated check).
How can you recognize a problem with corn diseases that affect seeds and seedlings? Disease damage may appear to be similar to some environmental stress, but general and specific symptoms can help you diagnose a disease problem. General effects of corn seed and seedling disease are reduced emergence, slow growth and stunting in a random or circular pattern, wilting, chlorosis/yellowing, and postemergence damping-off. Specific symptoms of seed and root infections include rotted seed and seedlings before or after emergence; red/yellow discoloration of leaves; complete or partially rotted roots with firm or soft brown reddish to gray lesions or decay; discolored and soft coeleoptile; death of leaf tips; wilting; and sunken, discolored lesions on mesocotyl. Leaf spots and streaks can also occur, resulting from anthracnose and Stewart's wilt infections.
What are the pathogens/diseases that affect corn seeds and seedlings, and what is the source of these pathogens? Some common genera of "fungal" pathogens that cause one or more of the preceding symptoms are Diplodia (Stenocarpella), Pythium, Rhizoctonia, Fusarium, Colletotrichum, and Penicillium. Two bacterial pathogens that can affect corn seedlings are Erwinia [=Pantoea] (Stewart's wilt) and Pseudomonas (holcus spot). In addition, don't forget about nematodes, which also damage corn seedlings, especially in sandy soils. The source of these pathogens is typically the soil-infested residue remaining from previous years' crops and infested seed. Recently, we checked 10 seed lots for fungal infestation, and we frequently isolated Fusarium, Aspergillis, and Penicillium from surface-disinfested seed.
What conditions favor these pathogens? The frequency of infection and importance of these pathogens will vary and depend in part on location, seed quality (cracked or infected seed), soil temperatures less than 55°F, wet soil, soil compaction, slow emergence and growth, hybrid/inbred, fertilizer burn, herbicide injury, crusted soil, high temperatures (Penicillium infection), and high populations of flea beetles (Stewart's wilt). Common favorable conditions for seedling diseases are cool, wet, and compacted soil and poor seed quality. For example, Pythium is a common soil fungal-like pathogen that causes seed and seedling rots, and is favored by damaged seed and wet and cool soil conditions.
Which fungicidal seed treatments are most commonly used on corn, and are any new products available? Most corn seed sold in Illinois is treated with fungicides to provide protection from seed and seedling diseases. However, these chemicals are most effective for only about 2 weeks after planting, depending on soil water content and temperature. The primary fungicidal seed treatments used are of two main groups. The first group (ApronXL, Allegiance, and Apron) is most effective against Pythium. The second group of fungicides (Maxim and Captan are common examples) protect against the other fungi.
A new fungicidal seed treatment was labeled for corn in February 2003. The U.S. EPA approved the Syngenta Protege label as a seed treatment for field, seed, and sweet corn. Due to the late registration, treatment of commercial seed has been limited in 2003. The active ingredient in Protege is azoxystrobin, the same active ingredient in Quadris foliar fungicide (Syngenta) and one of the active ingredients in SoyGard soybean seed treatment (Gustafson). Azoxystrobin is reported to have greatest activity against Rhizoctonia, Fusarium, and Penicillium, and research is under way in Illinois and elsewhere to improve our understanding of its activity and efficacy against various corn seed and seedling diseases.--Dean Malvick