Late Season Soybean Diseases

 

 

 

 The fungus that causes stem canker infects early in the season, but doesn’t show symptoms until well into the reproductive stages of growth. Early indications of this disease include darkening of the stem, usually near a node. Sometimes the entire bottom part of the plant will be killed, but the upper part remains green or vice versa.  Ultimately, the whole plant can be killed. University of Nebraska has some nice pictures here.

 

There is some resistance available among bean lines, so if you have a significant problem this year, talk to your seed dealer about varieties. Rotation to a non-bean crop can also help, but fungicides have not proven consistent for control of this disease.

 

 

;Sudden death syndrome of soybean (SDS) has shown up in isolated cases around the state. The symptoms of this disease are pretty striking, but can be mimicked by other diseases like brown stem rot or injury from chemicals. You will first notice yellowing progressing to browning between the leaf veins, while the vein area stays green. If you see these types of symptoms in your beans, check out this Purdue University bulletin which goes on to explain more about the disease and what you should do if you find it.

 

This is a soil borne disease affecting the roots caused by a fungus (Fusarium virguliforme). This fungus can attack the roots at any time in the season, including long after many seed treatments may have lost efficacy.

 

Even though this is a root disease, the first symptoms you’ll see are in the leaves. Early on you may notice some distortion of new growth, but you’ll definitely react to the next stage which shows itself as a distinct yellowing then browning between the veins resulting in a striking contrast of coloration (Figure 1 in the Purdue University bulletin). This symptom alone is not enough to be sure you have SDS since Brown stem rot (BSR) and a few other diseases can show this--you’ll also need to slice open the stem lengthwise and observe the tissue there. In a plant affected by SDS, the tissue on either side of the pith (the very middle part) will start browning. Meanwhile, a plant affected by BSR will show browning inside the pith itself and the areas to either side will remain white. Further SDS affected plants may also have root rot and a grayish discoloration (sometimes with a bluish cast to it) of the tap root. The blue color is the fungus itself, but often disappears as the root dries out.

 

SDS becomes a problem first in areas of high compaction and poor drainage. This is why it appears in a spotty pattern in the field. It will not become a problem every year, since environmental conditions won’t always be right. However, once you observe SDS be sure to select a bean variety with resistance to SDS to plant in that field each year. The fungus can survive for long periods of time.

 

While fungicides are not generally helpful for this disease, and seed treatment efficacy is not yet known, we do have resistance available in our soybean lines that is quite good. There is a strong link between the presence of this disease and infection by soybean cyst nematode. While we don’t seem to have a historical problem with this nematode in PA, it might be worth sending a soil sample for testing if you have any suspicions.

 

 

Conditions lately have been good for the fungus that causes white mold. Sclerotinia sclerotiorum is a soilborne pathogen that will cause white fluffy growth on the lower stems of beans, and ultimately form hard, black structures that may survive for years. Under a thick canopy with little air flow is where this fungus thrives, so get down low and scout as many locations in your field as possible. It spreads slowly on its own, but once you’ve got it in a field it is incredibly difficult to get rid of it.

 

Anne Dorrance, Plant Pathologist at Ohio State, advises us to: “Monitor fields. First, make note of the variety that was planted, if infections are above 20% incidence (more than 20 plants out of 100 are infected), then drop this variety from your lineup. Second, harvest these fields after all of the other fields are harvested.& …the approach here is to prevent contaminating additional fields.”&

 

There are some fungicides labeled for white mold; however, it is challenging to get a spray to the lower stem where it is needed past the R3 growing stage.

 

 

We think of Phytophthora mostly as an early-season, cool, wet weather-favored pathogen. While this is accurate, sometimes we only see the symptoms well after the initial damage to the plant has been done. In older plants you may see wilting and yellowing in the upper canopy as a result of an infection that took place in the roots shortly after planting. Only now is it showing up as the plants become more water stressed.

 

When you see these foliar symptoms, pull up a few plants and observe the lower stems and roots. In a plant affected by Phytophthora, you may find a poor or necrotic root system along with lesions that may progress up the stem.