The host range for Pythium spp. is extremely wide. Vegetable crops commonly infected include beans, cucurbits, peppers, southern peas, strawberries, and tomatoes.
Pythium typically attacks roots causing damping-off, seedling blights, root rots, and wilting of affected crops. In some instances, Pythium may affect the above ground portions of crops. P. myriotylum and P. aphanidermatum are generally most abundant in Florida because they are adapted to high soil temperature. The optimum temperatures for their growth and infection of plants range between 86°F and 98°F.
One of the characteristics of tissue infected with Pythium spp. is the presence of water soaked or greasy appearing tissue. This is in contrast to the orange to red to dark sunken lesions caused by Rhizoctinia solani. Infection with Pythium spp. also causes wilting of numerous crop species. Plants affected by Pythium root and stem rots commonly exhibit yellowing of the lower leaves.
In small plants planted thickly, such as greenhouse transplants, Pythium can infect and colonize the plants with the result that the entire plant is destroyed. Look for water-soaked tissue in this situation. It also is common to see white mycelial growth in such situations. Excess fertilizer, flooded soils, insect feeding, and nematode feeding also may contribute to dysfunctional roots. For accurate diagnosis, it is best to submit samples to a reputable diagnostic laboratory.
Survival And Spread
Pythium thrives in moist soils and multiplies and spreads rapidly under wet conditions. Although Pythium is capable of producing several spore types, zoospores and oospores are most important. Zoospores are mobile. They are produced rapidly and in great numbers and contribute to the organism’s ability to cause disease almost overnight. Zoospores may be detected within half an hour after a site is flooded and can “swim” for up to 30 hours and move three or more inches through soil.
Oospores are extremely durable and can survive in soil and infected crop debris for more than 10 years. A number of broadleaf and grassy weeds may host Pythium spp. and serve as important sources of inocula.
Resistant cultivars do not exist, so control of Pythium depends on a variety of tactics. Crops should be planted on raised beds in well-drained soils. Pre-plant soil fumigation is effective if applied correctly. Soil solarization has successfully suppressed Pythium in some cases. Fumigant formulations containing chloropicrin are most effective in providing control. If solarization or a soil fumigant is used, raised beds are important since fumigated soil has minimal or no beneficial organisms to compete against pathogens. Control is at best temporary; as under the right conditions, zoospores from un-fumigated soil may readily re-infest the treated bed.
A number of chemical treatments are available for damping-off control. Seed treatments containing mefenoxam (Apron, Syngenta) work best. Fungicidal drenches like Ridomil Gold (mefenoxam, Syngenta) are effective for the suppression of seedling blights and root rots if applied before infection occurs.
By Gene McAvoy, Regional Vegetable Specialist IV, University of Florida/IFAS