The following is an update on the diseases we are seeing around the state at this point in the crop year and general thoughts on disease management. Dr. Jason Kelley and Dr. Travis Faske also contributed information for this update.
Frogeye leaf spot on soybean. Frogeye was found in Armor DK 4744 Monday at Rohwer Station. The soybeans were at growth stage R3 and the area of the field found to have active lesions was approximately a few meters in diameter. By Tuesday afternoon, and after isolated rain and cloudy weather, more lesions were observed in the field (Fig 1.).
This is an early detection of the disease but doesn’t necessarily indicate how severe frogeye will be this year. More information on the levels of frogeye on susceptible and moderately susceptible cultivars will be posted to this blog when the disease is found in other areas of the state. Varieties with known susceptibility to frogeye leafspot should be actively scouted this week.
“Mystery root disease” present in soybeans. I have visited a number of fields in Drew, Desha, and Chicot Counties this year and last year that have a root rot disease. Plants appear stunted and the leaves are yellow between the veins. Some plants are dead or exhibit yellow foliar symptoms with necrotic “flashing” (Fig. 2). When the plants are removed from the bed, if the taproot is extracted and still attached to the plant, it is dead or dying, soft, and black (Fig. 3).
Sometimes much of the taproot is absent. The distribution of the disease varies from sporadic to a few consecutive diseased plants every 10-15 yards of row (approximately). This disease has been observed on soybeans in Louisiana and Mississippi and the symptoms in each state are very similar. Yield loss in Arkansas fields in 2014 was calculated to be 30% on affected plants. However, due to the sporadic nature of the disease in most fields, the yield lost in the entire field due to this disease appeared to be negligible.
Experimentation is ongoing to determine the causal organism, characterize the disease, and determine management options. A cooperative effort is being made between all three states to better understand this disease. Because this is a disease of the roots, fungicide sprays are not recommended and would likely not be effective at this time.
This disease is likely being incorrectly identified as sudden death syndrome caused by a fungus, Fusarium virguliforme. While the foliar symptoms may appear similar to some, the flashing of SDS typically occurs later in the year during late reproductive stages of development.
The root symptoms are also different as the black growth on the taproot is not commonly associated with SDS. Additionally, another disease of soybean, which has been found in Arkansas, is black root rot caused by Thielaviopsis basicola. This particular disease was found in 2008 in a field in Phillips County and described in the scientific literature in 2010.
The same fungal pathogen also causes black root rot in cotton. Black root rot is found in cooler wet soils, typically earlier in the year soon after the plant emerges. Numerous plant and soil assays have been conducted in Arkansas and Mississippi soybeans with little to no evidence of association by T. basicola with the “mystery root disease”.
Bacterial diseases in grain sorghum. At least two bacterial diseases have been active in grain sorghum. The first two images are of a disease called bacterial top and stalk rot caused by a bacterium in the genus Erwinia. The disease was found in Drew County and in another field close to Marianna. In the field, the tops of the plants die with four or five leaves seemingly unaffected (Fig. 4).
When the stalk is split open longitudinally, the center is deteriorated with purple/red water-soaked tissue and has a strong odor (Fig. 5). In fields that the disease has been found, symptomatic plants are sporadic and randomly distributed. In the fields we have seen, few plants relative to the total population are affected and so the economic importance to this point is likely low. The second disease is bacterial streak caused by Xanthomonas holcicola (Fig. 6).
This disease is fairly common on grain sorghum in a year when rainfall has been abundant. Typically, this disease is cosmetic and responsible for little yield loss (if any). There are no chemical treatments that will be effective for either disease.
Worth mentioning in this post, is that fungal foliar disease on grain sorghum has been light to this point in the season. However, some fields have been found to have a disease called sooty stripe caused by the fungus Ramulispora sorghi (Fig. 7). Sooty stripe lesions have a reddish-purple border with a yellow halo around the lesions with dark superficial sclerotia on the leaf. The sclerotia can be wiped off like dusty grains thus the “sooty” appearance.
For foliar disease management in grain sorghum, decisions on fungicide application should be made based on the timing of the disease onset, varietal susceptibility, and the weather. Generally speaking, humid, wet and warm is a combination of environmental conditions that will favor the spread of a foliar disease.
On a susceptible variety, disease will spread and a fungicide application may be required to protect yield. Remember, late season disease can be impressive but the opportunity for yield loss lessens as plants approach maturity. In trial work from around the state, fungicide application has typically resulted in keeping 3-5 bu/a in yield when compared to the untreated control in the presence of active foliar disease.
These results are more often a numerical trend and not statistically significant, indicating high variability among treatments with respect to yield. One should strongly consider the application cost of a fungicide before applying given this demonstrated variability.
Northern corn leaf blight. This disease has been found in a number of counties. Caused by the fungus Exserohilum turcicum, the typical symptom is a large cigar-shaped lesion oriented lengthwise along the leaf (Fig. 8). Conidia form in the lesion and are dispersed by wind and water to infect other plant tissue.
In susceptible hybrids, numerous lesions can form and begin to coalesce, creating large necrotic areas on the leaves reducing functional leaf surface area. Since the disease is favored by cooler wet weather, the hotter and dryer weather in many areas of the state of the last few weeks may have slowed disease.