Posts Tagged ‘soybean downy mildew’

Corn and Soybean Disease Update – August 12, 2011

Friday, August 12th, 2011

Bob Mulrooney, Extension Plant Pathologist;

Gray leaf spot is increasing in irrigated corn but it is too late to affect yield. This late in the season we often see an increase in gray leaf spot as well as Northern corn leaf blight.

We have seen several samples of ears with the milk line half-way up the kernels with random discolored kernels. This is Fusarium ear rot caused by Fusarium moniliforme. The fungus travels down the silks and infects the individual kernels. Often white fungal growth can be seen as well. Hybrids vary in their susceptibility and infection is favored by hot, dry weather. When severe the whole ear can be whitish. Often most of the fungus growth is limited to the tips of the ears. If grain is to be stored it is important to dry it sufficiently to prevent growth of the fungus to prohibit growth of the mycotoxin, fumonisin.

Individual random kernels and some tip infection by Fusarium moniliforme.

Despite the hot, dry weather downy mildew is showing up in full season irrigated soybeans now. Varieties vary in their resistance, but this fungus disease has never been yield limiting or damaging here. The disease produces numerous small yellow spots on the upper leaf surface and a tuft of grayish fungal growth on the corresponding lower leaf surface.

Downy mildew on the upper leaf surface of soybean caused by Peronospora manshurica.

Grayish tufts of the downy mildew fungus on the lower leaf surface.

Soybean Leafspot Diseases are Showing Up

Friday, July 23rd, 2010

Bob Mulrooney, Extension Plant Pathologist;

Several leafspots are beginning to appear on susceptible soybean varieties. These include Septoria leafspot as well as downy mildew. If you have good plant growth and a dense canopy after the recent rains and if the fields are irrigated both diseases could be present. Neither disease is thought to be yield limiting here on Delmarva.


Downy mildew on soybean showing the yellow irregular spots on the upper surface of the leaf


Closeup of lower leaf surface showing the small tufts of tan fungal growth of the downy mildew fungus that could be seen with a 10X hand lens.

Last chance to register for the upcoming Soybean Cyst Nematode Workshop, August 3. There are still spots open if you want to attend. See the announcement at the end of the newsletter.

Soybean Disease Update – July 31, 2009

Friday, July 31st, 2009

Bob Mulrooney, Extension Plant Pathologist;

Septoria brown spot and downy mildew continue to be the most common diseases seen at the present time. In areas that have had little rainfall and spider mite injury is present be on the lookout for Alternaria and Phyllosticta leafspot on stressed soybeans. Both fungi are weakly parasitic but infect drought stressed and mite infested soybeans.

alternaria leafspot on soybean

Alternaria leafspot on mite infested soybean leaf

Soybean Rust
On July 28, soybean rust was found in Assumption Parish, Louisiana, in a soybean sentinel plot. Soybean rust scouting continues in the U.S. and Mexico. In 2009, soybean rust has been found in five states and 32 counties in United States, and in two states and five municipalities in Mexico. There is no active soybean rust in Mexico at the present time.


Soybean Disease Update – July 24, 2009

Friday, July 24th, 2009

Bob Mulrooney, Extension Plant Pathologist;

Soybean Rust Update
On July 21, soybean rust was reported on kudzu from Escambia County in far west Florida. This site has been positive in previous years. On July 20, soybean rust was reported from Avoyelles Parish, Louisiana on soybean. On July 10, soybean rust was reported on kudzu in Columbia County, Florida. Soybean rust scouting continues in the U.S. and Mexico. Soybean rust scouting in DE will begin in August.


Downy Mildew
Downy mildew in soybeans is now being seen. Soybean downy mildew is caused by the fungus, Peronospora manshurica which only infects soybeans. The fungus causes irregular yellow spots on the upper leaf surface and a tuft of gray fungus growth on the corresponding lower leaf surface. Varieties vary in their level of resistance to this fungus. As best we know here downy mildew rarely, if ever, affects yield. In heavy infections seeds can become infected and have a coating of the reproductive structures of the fungus (oospores) over them. Fungicides are not recommended.

soybeandownymildewDowny mildew on soybean

Downy Mildew on Soybeans

Thursday, July 31st, 2008

Bob Mulrooney, Extension Plant Pathologist;

Downy mildew is showing up in Delaware soybean fields with the increased humidity and shower activity. Downy mildew is a common disease of soybean that occurs wherever soybean is grown. Fortunately, however, it rarely affects soybean productivity in Delaware. The downy mildew fungus, Peronospora manshurica, is biotrophic, which means it can only grow in association with the soybean plant. Because of this very close relationship with the soybean plant, it is capable of rapid genetic change in response to genetic changes in soybean. Thirty-three races are described for P. manshurica and the number of described races will likely increase as research continues.

Downy mildew appears on the upper surface of young leaves as pale green to light yellow spots which enlarge into pale to bright yellow spots. The spots look slightly gray and fuzzy when viewed from below, especially during periods of high relative humidity. (See the following photos) Younger leaves are more susceptible to downy mildew than older leaves. Occasionally when conditions are very favorable for disease and the variety is susceptible seed infection can occur which produces a dull white crusty coating of spores on the seed. Fungicide control is rarely needed.


Downy mildew on soybean (upper surface)


Downy mildew (lower surface)