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

Cultural Control:
1. Destroying crops at the end of harvest and controlling weeds, especially lambsquarters,
chickweed, and plantain, is important for reducing the number of overwintering
pupae.
2. Deep plowing can bury pupae and reduce the number of emerging flies the following
spring.
Materials Approved for Organic Production:
Since spinosad (Entrust) penetrates leaves to some extent, some farmers claim that it is effective
against leafminers; however, at the time of this publication, reports of efficacy studies have
not been found.
WEBWORMS
Webworms are more of a problem in the south and west than they are in the Northeast, but
occasionally, the garden webworm (Achyra rantalis) or the alfalfa webworm (Loxostege sticticalis)
reaches levels of concern in New York and New England. Webworms begin feeding on the
undersides of leaves. As they grow larger, they eat through the leaves, creating large notches
or holes. As the name implies, webworms spin webs among the leaves (Photo 3.3). Damage
from either young webworms or small infestations may go unnoticed, but as the larvae mature,
they can consume leaves rapidly and destroy a whole crop. Webworms overwinter in soil
as either 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 occurs when plants are small.
Cultural Control:
To keep local populations small, management of weed hosts is important.
Materials Approved for Organic Production:
Bt and spinosad give good control. Early detection is important for preventing significant damage.
II. DISEASE CONTROL_________________________________
DISEASES CAUSED BY BACTERIA
SCAB (Streptomyces scabies)
Streptomyces scabies is a common soil inhabitant that can persist indefinitely with or without
a host. When host crops are grown, the Scab pathogen populations increase, resulting in an
increase in disease severity in subsequent years. It causes rough, tan to brown, raised, corky
lesions on the root surface of beets, radishes, turnip, parsnip, carrot, and potato tubers. Spinach
and chard are not affected. This pathogen belongs to a group of organisms called actinomycetes,
which are filamentous bacteria and include many beneficial species commonly present
in soil. Streptomyces scabies produces asexual spores in chains. Because of the filamentous
growth ability of actinomycetes, this pathogen can spread to reach new nutritional sources by
growing between soil particles. Scab is a common problem in high pH soils, but it is usually
not severe or of economic importance on Chenopod crops.
Cultural Control:
There are no cultural practices other than lowering soil pH.
Materials Approved for Organic Production:
None.
28 Organic Resource Guide