Soybean vein necrosis virus was first confirmed in Iowa last summer. "Last year we did not see this disease show up until August," says Daren Mueller, an Iowa State University Extension plant pathologist. "But this year we identified this disease appearing here during the second week of July. And we identified it in several locations in Iowa. It is not known yet if these earlier symptoms that we are seeing this year may increase the chances of yield loss. We will continue to monitor and provide updates."
The virus belongs to the tospovirus group, which is vectored or spread by thrips and possibly other insects. Mueller says symptoms often begin as chlorotic (light green to yellow) patches near the main veins of the soybean leaf, and these patches may enlarge and eventually become necrotic or brown areas (see accompanying photo--Figure 1). The veins may appear clear, yellow or dark brown. The browning of the veins may be especially noticeable on the lower leaf surface (see the other accompanying photo--Figure 2), but this may not always occur.
Plant disease experts have no management recommendations for this new disease
Currently, there are no management recommendations for this disease, says Mueller. Other crop disease systems that include thrips and tospoviruses, including tomato spotted wilt virus, focus on resistance and management of the vector. Because of the newness of this disease, there are no known sources of resistance in soybeans—so you can't really choose a soybean variety to plant that is resistant to soybean vein necrosis virus.~~~PAGE_BREAK_HERE~~~
What about applying an insecticide to kill the thrips or insects that spread this disease in soybean fields? Mueller says insecticide application should only be considered in fields with a known risk of yield loss.
Figure 1. Foliar symptoms of SVNV on soybean
Figure 2. Browning of the veins on the lower leaf surface
Watch for other soybean diseases to possibly show up this summer
Soybean diseases are most common when soil is very wet in the first few weeks after planting, especially in heavy, poorly drained, compacted or high-residue fields. If field conditions remain cold and wet, experts recommend scouting untreated soybeans for seedling diseases such as Phytophthora, Pythium, Fusarium, and Rhizoctonia.
We had wet conditions earlier this spring—but during the last month we've had very dry weather—from mid-June until today which is July 22. So diseases may not be as bad in soybeans this summer as was first thought. But keep an eye on your fields for symptoms.
Pythium and Phytophthora are associated with wet, waterlogged conditions. Pythium is more active in cooler soils (50 to 60 degrees), while Phytophthora is more active in warmer soils above 60 degrees. Pythium may be the first soybean disease found in the growing season. Symptoms are commonly referred to as "damping off", with seeds germinating but rotting before emergence, or small seedlings that wilt and die.
Phytophthora symptoms include a dark discoloration of the stem, usually beginning at the soil line. Diseased tissues quickly become soft and water-soaked. Wilting and plant death may soon follow. Many soybean varieties now have the Rps 1k gene for Phytophthora resistance and field tolerance.
Rhizoctonia is more often found in late-planted soybean fields. It appears as the weather becomes warm (around 80 degrees) and is more common in wet to moderately wet soils where germination is slow or emergence is delayed. It is characterized by a shrunken, reddish-brown lesion on the hypocotyl at or near the soil line. The infection may be superficial, causing no noticeable damage, or may girdle the stem and kill or stunt plants.
Fusarium causes discoloration and decay of the lower taproots and lateral roots. The disease is most common in poorly drained, waterlogged soils. However, if the pathogen is present, it can also infect drought stressed plants in dry soil conditions. Fusarium is common in many soils but often goes unnoticed in the presence of Pythium, Phytophthora and Rhizoctonia symptoms.