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

1. Minimize leaf wetness, and maximize ventilation.
2. High calcium levels in the soil that result in a calcium to phosphorus ratio of two or
higher in the leaf petiole aids in reducing susceptibility of tomato plants.
Materials Approved for Organic Production:
Copper may be effective. Begin application before the canopy becomes dense.
LATE BLIGHT (potato, tomato) Phytophthora infestans
Late blight (Photo 7.20) is caused by a water mold pathogen and is a serious disease of
both potato and tomato. It is infamous as the cause of the Irish potato famine. It can quickly
defoliate plants and cause fruit rot in tomato and tuber rot in potato. Spores can be carried
long distances in the wind, and the disease can spread rapidly. In the Northeast, the pathogen
overwinters only in living plant debris, most commonly on seed potatoes or unharvested and
cull potatoes. As greenhouse and high tunnel tomato production becomes more common,
overwintering may occur on infected tomato tissue that survives the winter. Imported
solanaceous transplants, including tomato and petunia, may harbor late blight.
Cultural Control:
1. Destroy cull potatoes and control potato volunteers in all fields.
2. Do not allow infected greenhouse or high tunnel tomatoes to survive the winter.
3. Use drip, rather than overhead, irrigation in order to keep the foliage dry. Alternatively,
overhead irrigate early in the morning (i.e., before dawn), so the plants are dry early in
the day. The key factor is to keep the period of leaf wetness to a minimum.
Materials Approved for Organic Production:
Copper products give fair to good control but must be applied often and thoroughly.
LEAF MOLD (greenhouse tomato) Fulvia fulva
In the Northeast, leaf mold has historically been a disease of high tunnel and greenhouse
production; however, it is now becoming common in field production as well. It is a disease
that is usually a problem only under highly humid conditions. The initial symptoms are light
green to yellowish spots on the upper surface of the leaf (Photo 7.21), with a dark green mold
on the undersurface of each spot (Photo 7.22). Later, the spots coalesce, and leaves wither and
drop from the plant. Usually, only the foliage is affected. Older leaves are affected first, so if the
disease arrives late in the season, it does not affect yield. If it strikes early, the loss of foliage will
reduce yield. Blossoms, petioles, and fruit may be affected during severe outbreaks.
Cultural Control:
1. Sanitation is important. After harvest, remove all crop residue, and steam the
production area for at least six hours.
2. Minimizing periods of leaf wetness reduces the severity of leaf mold. Avoid wetting
leaves by overhead irrigation or dripping condensation. Maintain good ventilation.
3. There are many resistant varieties of tomatoes suitable for high tunnel and greenhouse
production.
Materials Approved for Organic Production:
None known.
PHYTOPHTHORA BLIGHT (pepper, tomato, eggplant) Phytophthora capsici
Phytophthora blight is a serious problem of peppers in warmer parts of the Northeast and
will attack tomato, eggplant, cucurbits, and beans. Plants collapse as the pathogen attacks
the roots and crown and girdles the stem. Fruit will rot where they touch the ground or from
spores carried in splashing water (Photo 7.23). Fruit lesions develop as dark, water-soaked
areas that spread and become coated with the white spores. P. capsici overwinters in the soil.
Organic Resource Guide 63