Symptoms: Symptoms usually appear about 7 days after bloom, with a white cottony growth on the flower surface. Later, irregularly shaped water-soaked spots appear on stems, then on branches and leaves. Spots enlarge to become soft watery, slimy rots. Cottony fungus grows over these parts, then small, hard, black structures develop. Leaves above the stem infections become yellow then brown and often drop. The plants may wilt and the canopy may appear more open due to the loss of leaves. One or more stem infections can kill a plant. When leaves are infected, they dry out and bleach to a pale brown or white, much paler than a normal dead leaf color. The fungus can grow into the pods, or they can be infected by splashing spores. The white cottony fungal growth can completely cover the pods. Infected but symptomless pods harvested with healthy pods provide the beginning for severe storage/transit loss.
Prevention: It is important to keep the leaves and the soil surface dry and the air around the plants circulating. Choose cultivars that have an open canopy and upright growth habit. Space rows widely, and orient rows so the prevailing winds blow down the rows. Avoid planting in low areas where soil remains wet for long periods, or near windbreaks where leaves won't dry. Irrigate only when necessary, and irrigate early in the day so leaves and soil can dry. Excessive amounts of nitrogen fertilizer should by avoided. Refrigerate harvested pods at 45-50 F to avoid storage losses. Resistance is available for this disease, and is usually related to upright growth habit. See current recommendations for chemical control measures.
Downy Mildew is caused by Phytophthora phaseoli. It affects Lima Bean. The disease was first reported in 1889 in CT, and is now found worldwide. It primarily causes molding of the pods, especially those lying close to the soil. It may cause seedlings to damp off as well.
Symptoms: On leaves, symptoms start as white spots that enlarge and eventually cause the leaves to wilt and die. Young branches, flower branches, and buds are distorted and covered with white downy or cottony mold. Older leaves are usually not heavily covered with mold, but the leaf veins may be purplish and twisted. White downy or cottony mold appears in patches on pods. The invaded areas may be bordered in reddish-purple. The pods shrivel, die, and turn black. Seeds may be invaded as the fungus grows through the pod. They usually remain attached to the plant.
Identification of disease: White downy or cottony mold
Prevention: Use disease free seed. Plant in well drained soil. Rows should run the direction of the prevailing wind to increase air circulation and promote leaf drying. Crop rotations can be helpful. Plow under debris after harvest. Resistance is available for this disease. See current recommendations for chemical control measures.
By Pamela S. Mercure, IPM Program Assistant, University of Connecticut
Hall, R., and J. R. Steadman. White Mold. pp. 28-29 in Compendium of Bean Diseases. R. Hall, ed. APS Press, St Paul, MN. 1991.
Sherf, A.F. and A. A. MacNab. Vegetable Disease and Their Control. John Wiley and Sons, New York. 1986.
Schwartz, H.F. Downy Mildew. p. 19 in Compendium of Bean Diseases. R. Hall, ed. APS Press, St Paul, MN. 1991.
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