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

Cultural Control of Root Rots:
1. Avoid using old seed with low vigor.
2. Deep plowing to bury root debris has been shown to reduce the disease in some
studies.
3. Avoid wet soils.
4. Depending on which species is present, temperature may be a factor. There are
some species that do well in cool soil and some that do well in warm soil. Personal
experience may help to determine if one or the other is more prevalent in a given area.
5. Resistant varieties have been developed for some root diseases, especially Fusarium
wilt. A complicating factor is that there are many races of each Fusarium species, and
resistance to one race does not mean the crop will have resistance to another. Most
modern varieties of peas are resistant to race 1 of Fusarium oxysporum f. sp. pisi. Race
2 causes a disease called “near wilt,” which may be the most common problem in the
northeast at this time. Diagnostic labs may be able to determine which race is present
in the soil. Work with seed companies to find varieties resistant to that race.
6. Crop rotation to grass green manures, cereal crops, pasture, or grass hay crops
may reduce soil infestation, increase soil organic matter, and improve soil structure
to reduce disease. However, to avoid problems with seed corn maggot, do not
incorporate organic matter immediately before planting. Plant seeds shallowly in
warm, moist (but not wet) soils to speed germination.
Materials Approved for Organic Production:
Biological control agents may be useful as seed treatments. For example, Actinovate AG
(Streptomyces lydicus), Mycostop (Streptomyces griseoviridis), and T-22 HC (Trichoderma
harzianum) are labeled for Pythium (see Material Fact Sheets for details and efficacy reports).
POWDERY MILDEW (Erysiphe pisi)
Powdery mildew is a very common disease, but in most years, it arrives late enough to not
severely reduce yields in peas (except for late production) and is not a significant problem in
beans. This genus has a wide host range, but there are different species that are specific to
certain crops (e.g., only the E. pisi causes powdery mildew on peas). In years when powdery
mildew is severe, it will reduce yield in both weight and number of peas per pod.
Erysiphe is an unusual fungus because it thrives in warm, dry weather, needing only moderate
dew in the evening to germinate and grow. In fact, rain and heavy dew wash spores off plants
and are deleterious to spore survival.
The first symptoms occur on the upper surfaces of older leaves, where light-colored spots
appear and eventually turn white and powdery (Photo 5.8). Small, black structures called
cleistothecia form in mature lesions. The fungus overwinters as sexual spores in cleistothecia
on infected plant debris and may survive in seed. Spread of the disease during the growing
season is by conidia spores blown in the wind.
Cultural Control:
1. Cleaning up crop debris and using clean seed is the first line of defense.
2. Resistant varieties are available.
3. Early production helps to avoid the problem.
4. Overhead irrigation can help to slow disease progression.
Materials Approved for Organic Production:
Sulfur is effective if used early in disease development. See Fact Sheet.
GRAY MOLD (Botrytis cinerea)
Gray mold affects most vegetable and fruit crops, including peas and beans. The fungus is
ubiquitous, and cool, moist conditions favor disease development. Botrytis usually infects
senescent tissue first and spreads if conditions remain favorable. During periods of leaf
wetness, dying petals, cotyledons, sepals, and older, lower leaves may become infected by
Organic Resource Guide 47