Nematode Supression by Cover Crops

Crop Rotation and Cover Crops
Suppress Nematodes in Potatoes

When Russ Ingham talks about his research on nematode-suppressive cover crops, you
can tell he's excited about the results of his work. Ingham, an Associate Professor in the
Department of Botany and Plant Pathology at Oregon State University, is studying the
effects of various cover crops on the suppression of the Columbia root knot nematode
(Meloidogyne chitwoodi ), a serious pest of potatoes in the Pacific Northwest. According
to Ingham, his goal is to develop strategies for controlling nematodes without the use of
fumigation.

"Fumigation can be an effective control," he says, "but it is expensive and has undesirable
impacts on non-target organisms in the soil.

"Because of the broad spectrum effects of fumigation," says Ingham, "many beneficial
soil ecological processes, such as nutrient cycling and biological control, are disrupted."

Ingham's research at the Hermiston Agricultural Research and Extension Center has
focused on the integration of crop rotation strategies and cover crops with the use of non-
fumigant nematicides.

In 1991, a replicated experiment was established comparing several combinations of
cover crops and non-fumigant nematicides with the "control" of wheat stubble.
Following wheat harvest, cover crops were planted in August and irrigated up. A warm-
season grass, the sudangrass cultivar (Trudan 8) grew rapidly in the fall and was killed by
frost in early November. The cover crop was then rototilled into the soil as a green
manure crop. A second cover crop, rapeseed (cultivar Humus), grew through the winter
and was incorporated in March, approximately 30 days prior to planting potatoes.
Ingham noted that rapeseed has well known allelopathic qualities which can suppress
potato growth if incorporated into the soil too close to planting. Potatoes were planted in
April and grown using standard cultural practices of the area.

Nematode population levels in the soil were determined on August 10 and percent culled
potatoes determined at harvest. To standardize data analysis, potato researchers consider
a potato tuber a cull if it contains 6 or more nematode stings.

Data from 1992 (Table 1) show high numbers of root knot nematodes and percent culled
tubers in the both the wheat fallow check and the wheat fallow plus the nematicide
Mocap. Potatoes grown following the fall-incorporated sudangrass experienced similarly
high levels of culled tubers, but the nematode levels in the soil were dramatically
reduced. The addition of Mocap to the sudangrass cover crop treatment reduced
nematode numbers, as well as the percentage of culled tubers. The greatest reduction in
nematode populations resulted following the spring-incorporated rapeseed cover crop.
Although nematode numbers were similar between the cover crop treatment and cover
crop treatment plus Mocap, the addition of Mocap dropped the percentage of culled
potatoes into the range considered to be economically acceptable (typically less than 5%).

"There are a couple of mechanisms probably working here," says Ingham. "First, neither
sudangrass or rapeseed is a host for the Columbia root knot nematode. Using non-host
crops in the rotation is critical for reducing nematode populations."

"There's also some interesting compounds in these cover crops", says Ingham. "Trudan
8 sudangrass contains a compound called dhurrin.  As the cover crop is decomposed in