Insects and weeds are two of farmers’ biggest foes. So what happens when those enemies join forces? That appears to be happening with some winter annual weeds and soybean cyst nematode, said a Purdue University Extension nematologist.

Winter weeds like henbit and purple deadnettle are showing up in fields around Indiana, and both weeds are susceptible to soybean cyst nematode (SCN), said Jamal Faghihi.

The conventional thinking used to be that the active growth period for winter weeds did not coincide with SCN activity, Faghihi said.

“However, over the last couple of years we have closely observed winter weed emergence and have noted that henbit and purple deadnettle can emerge in the spring, late summer and fall,” he said.

“Emergence in the spring can occur as early as mid-March and generally ceases in late April. Late summer or fall emergence can begin in late August when we have cool wet weather conditions. So now, we have evidence that the life cycles of winter weeds and SCN do overlap and the potential exists for SCN population increases on winter annual weed hosts.

SCN is not physically active when soil temperatures fall below 50 degrees, Faghihi said. The optimum temperature for SCN is 75 degrees. At 75 degrees the nematodes require about one month to complete one life cycle—about 750 degree days.

“Winter weed growth is fairly abundant this year,” Faghihi said. “Recently, we were able to document and report the completion of at least one generation of SCN in the field.”

A Purdue weed science graduate student studying the relationship between winter weeds and SCN followed a life cycle of SCN. The student extracted newly developed cysts on roots of purple deadnettle plants in a field in southern Indiana.

“The winter annuals in Indiana typically germinate in late fall and mature in early spring,” Faghihi said. “During this time period, under normal conditions, the Indiana soil temperature seldom reaches and stays at the required temperature for SCN development. With warm September weather conditions, completion of an SCN life cycle on winter weeds is a possibility.

“Growers might have an extra incentive to spray for winter weeds this fall as part of their overall farm management and SCN population control, if they have fields with both purple deadnettle and SCN.”

Purdue researchers continue to study the correlation between winter weeds and SCN, in hopes of establishing a winter weed management recommendation to manage SCN.

“We have yet to accumulate enough data to be able to make a recommendation for the northern half of the state of Indiana but we are getting close to more definitive answers,” Faghihi said. “We might be able to predict the activities of SCN on winter weeds based on the number of degree days required for SCN to complete the life cycle. The accumulation of degree days in southern and northern Indiana will be different in different years.”

A companion research project hopes to determine whether early fall removal timing has an influence on SCN reproduction on purple deadnettle.

The Purdue research is supported financially by the Indiana Soybean Board and the U.S. Department of Agriculture-Cooperative State Research, Education and Extension Service.

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