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

soil either as pupae or larvae. In the spring, the moths emerge and lay eggs on the undersides
of leaves either singly or in short rows. There can be two or more generations per year, but the
first is the most damaging because it feeds when plants are small and vulnerable.
Cultural Control:
Weed management is important to keep nearby populations small.
Materials Approved for Organic Production:
Bt and spinosad will give good control. Early detection is important to prevent significant
damage.
TWO SPOTTED SPIDER MITE (Tetranychus urticae)
Spider mites have piercing mouthparts that rupture the epidermal cells of plants, causing a
stippled appearance. Celery is the only umbelliferous crop that is significantly damaged by
spider mites. In large numbers, they can cause the leaves to become bronze or silvery with a
fine silk webbing. Mites are favored by hot, dry weather. They are very small and difficult to
see with the naked eye. A hand lens is necessary to check for mites on the undersides of leaves.
Populations can build rapidly; each female can lay up to 100 eggs, and the cycle from egg to
adult can be completed in 7-14 days. If large populations are left unchecked, plants may loose
vigor, yellow, and die.
Cultural Control:
1. Spider mites have many natural enemies, including several species of predatory
mites. Avoid using broad-spectrum insecticides that may kill beneficial insects. Mites
are often localized in small areas of a field, so spot treatment may suffice and spare
beneficial insects.
2. Overhead irrigation or heavy rains may suppress mites.
Materials Approved for Organic Production:
1. Mites can be controlled with summer weight oils. Be sure to read the label and
follow carefully. Phytotoxicity is possible with some crops, especially when used in
conjunction with sulfur or copper fungicides.
2. Entrust is labeled but not shown to be effective at normal use rates. It has been
demonstrated to be somewhat effective at high rates (see Spinosad Material Fact
Sheet for references).
3. Insecticidal soap has been shown to offer fair control.
II. DISEASE CONTROL_________________________________
DISEASES CAUSED BY BACTERIA
BACTERIAL LEAF BLIGHT (Xanthomonas campestris pv. carotae)
Bacterial leaf blight affects only carrot. The first symptoms are small, yellow spots that expand
quickly into irregular, brown, water-soaked lesions surrounded by a yellow halo (Photo 9.6).
The centers of the spots may become tan and dry. Lesions are common on the leaf margin and
progress along the edge of the leaf. The infection will typically progress down the main leaf and
petiole veins. This progression of the disease distinguishes it from Alternaria and Cercospora
leaf blights (see below). Bacterial leaf blight can be an explosive disease under hot, rainy, and
windy conditions.
This bacterium is a common contaminant of carrot seed and can also persist on crop debris.
It does not persist in the soil once the debris is decomposed. Spreading occurs mainly by
splashing water, though it can also be transmitted by insects and farm machinery. Symptoms
appear when bacterial populations build to high levels. Persistent leaf wetness and warm
temperatures are needed for bacterial populations to grow.
76 Organic Resource Guide