Resource Guide for Organic Insect and Disease ... - Cornell University

Cultural Control:
1. Practices that promote good airflow and quick drying are the most important
management, e.g., wide rows oriented in the direction of prevailing wind, wide spacing
in rows, good weed management, irrigation timed not to increase the hours of leaf
wetness.
2. Rotation with non-host plants like corn helps. Since the sclerotia are very long lived
without a host in the soil, rotation must be long to be of value.
Materials Approved for Organic Production:
Coniothyrium minitans (Contans). Contans is a fungus that, once applied and incorporated
into the soil, attacks and destroys the white mold sclerotia. Follow the directions on the label
carefully because success is dependent on eliminating near-surface sclerotia that are likely
to germinate and produce apothecia and spores. Using enough of the material to reach all
of the sclerotia in the soil profile would be cost prohibitive. Contans requires 3 to 10 weeks
to effectively colonize and destroy sclerotia. Apply Contans to a Sclerotinia-infected crop
immediately following harvest at 1 lb/A, and incorporate the debris into the soil. Alternatively,
apply at 2 lb/acre to a crop right after planting, followed by shallow incorporation (or irrigate)
to about a 1 to 2 inch depth. After application, do not till deeply, or sclerotia that are deeper
than the Contans treated zone will be brought to the surface. To reduce survival of the
sclerotia, Contans should be applied after a crop with high levels of white mold infection.
PHYTOPHTHORA BLIGHT (Phytophthora capsici)
Beans are a host of this pathogen that has, over recent years, become an increasing problem
in peppers and many cucurbits. Phytophthora is not common in beans, but has been reported
over the past few years. Moreover, Phytophthora blight in cucurbits has been reported to be
worse when following a bean crop.
Initial infection comes from overwintering oospores that, when the soil is wet, germinate to
produce sporangia. Swimming zoospores are released from the sporangia, so plants in the
wettest areas of the field are generally infected first. In beans, large, water-soaked lesions
can occur on the leaves, stems, or pods. Pods shrivel and become leathery, later developing
a characteristic white, yeast-like growth typical of Phytophthora. Large numbers of sporangia
are produced, causing the disease to spread rapidly by water movement under wet conditions.
Phytophthora blight is most common in beans after severe rainstorms.
Cultural Control:
1. Avoid fields with a history of Phytophthora blight.
2. Crop rotation to non-host crops (such as sweet corn or crucifer crops) is recommended.
3. Arrange plantings so water movement does not potentially bring the pathogen from
an infested to a clean field.
4. Manage soil tilth to promote rapid drainage.
5. Subsoiling, chisel plowing, cover cropping, and any other practice that helps avoid wet
areas is recommended.
6. The pathogen can easily move from field to field with infested soil or runoff. Irrigation
water can be a source of inoculum.
7. Clean equipment and boots if used in an infected field, and always work in uninfested
fields before infested fields.
ANTHRACNOSE (Colletotrichum lindemuthianum)
Anthracnose is primarily a problem on dry beans. The pathogen overwinters in the seed and
in plant residue and, under damp conditions, can germinate and produce spores quickly. The
spores are windblown, and when there is incessant wet and windy weather, the disease can
become widespread and devastating. Lesions begin as tiny brown specks that develop into
dark brown spots with purplish rims and light centers (Photo 5.12). Lesions can develop on
any part of the plant, but they are most damaging when on the pod in snap beans or on the
seeds in lima and other dry beans because they reduce marketability. Although there are some
Organic Resource Guide 49