By Dennis O'Brien

A mystery about a disease that can destroy up to 15 percent of a cotton crop
in the southeastern United States has been solved by Agricultural Research
Service (ARS) researchers. The work could save cotton crops and prevent
unnecessary insecticide spraying.

In 1999, scientists reported an emerging seed rot disease that was
discoloring seeds and darkening fibers in cotton bolls in the southeastern
states, making the crops unmarketable. It quickly spread throughout the
southeastern Cotton Belt.

To study the problem, plant pathologists Gino Medrano and Alois Bell of the
ARS Cotton Pathology Research Unit, part of the Southern Plains Agricultural
Research Center in College Station, Texas, focused on the southern green
stink bug (Nezara viridula L.) as the disease-transmitting culprit.

In the greenhouse, they infected cotton bolls with suspect bacteria, using a
needle to penetrate the boll wall and mimic a stink bug's bite. The results
showed the needle created a pathway for the bacteria to enter the boll and
damage it.

The researchers used a strain of the bacterium Pantoea agglomerans in the
research, but other bacteria may also be involved in damaging cotton plants,
according to the scientists.

In other tests, Medrano and entomologist Jesus Esquivel of the ARS Areawide
Pest Management Research Unit in College Station showed why stink bugs
sometimes--but not always--cause extensive damage in cotton fields. By
infecting bolls at various stages, they found damage levels depend on when
infections occur in the fruiting cycle and on how long infection is allowed
to spread before harvest. Bolls infected three weeks after flowering are
resistant and undamaged. Younger bolls remain susceptible.

Farmers often spray insecticide to combat stink bug infestations. But
knowing mature bolls are immune to infections should help farmers decide
when to spray. Medrano also is developing a test kit that will offer
guidance by telling farmers if stink bugs in their fields are infested with
the pathogens that cause the seed/boll rot.

A report on Medrano's work appeared in a recent issue of the Journal of
Economic Entomology.

ARS is a scientific research agency of the U.S. Department of Agriculture.
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