US: Two destructive diseases of soybean spotted

He explained that symptoms of SDS that currently are being observed include interveinal chlorosis and necrosis of the leaves (veins remain green while the tissues between the veins turn yellow and then brown). “These symptoms look exactly like the foliar symptoms caused by a different disease, brown stem rot. Brown stem rot, however, causes internal browning of the pith in soybean stems while SDS does not affect soybean stems,” he said.

On SDS-affected plants, the leaves will fall off eventually, while the petioles will remain attached to the stems and branches. In some cases, Bradley said, a bluish-white mass of spores of the SDS fungus (Fusarium virguliforme) may be observed on the roots.

“Although the foliar symptoms of SDS are now being observed, infection by the SDS fungus occurred during the seedling stage, not long after planting. The symptoms that are now being observed are the effect of toxins that the SDS pathogen produces that are phytotoxic,” he said.

Cool and wet weather after planting along with recent rainfall received in parts of the state have been favourable for infection and disease development and are the reasons that SDS incidence is high in some areas this year, Bradley explained.

“The primary method of managing SDS is to choose the most resistant soybean varieties available. Some evidence has shown that high soybean cyst nematode (SCN) egg populations may also increase the likelihood of severe SDS; therefore, managing SCN populations through resistant varieties and crop rotation may also reduce the risk of SDS,” he said.

White mould has also been observed in fields located in the northern half of Illinois this year. “The appearance of this disease also is weather-related. Areas in the northern half of the state that were cooler and wetter than normal after soybean plants began to flower are the areas that are affected the most severely,” Bradley said.

“Unfortunately, once white mould signs and symptoms are detected in the field, fungicide applications generally will be futile as the damage has already been done,” he added.

Bradley said that growers with severe levels of white mould may encounter some discounts at the elevator this year for high levels of foreign matter. “Some sclerotia (dark survival structures produced by the white mould fungus Sclerotinia sclerotiorum) that are formed on plants may be similar in size to the seed, and will make their way to the hopper and eventually the elevator, where discounts may be received,” he explained.

Source: University of Illinois