Frogeye leaf spot (FLS) has been increasing in activity over the past few weeks in some fields with the highest severity located in central Arkansas. In this area, disease severity has ranged from moderate to severe on highly susceptible soybean varieties (Fig. 1). Alternately, disease severity has remained relatively low in fields planted with resistant varieties. Weather conditions have been favorable for disease development, which consists of cool temperatures (77 to 86ºF) and prolonged dew periods or light rain. Fields treated at mid-season (R3) with a fungicide may not have season-long protection on highly susceptible soybean varieties thus; these fields should be monitored closely when conditions favor disease development.
During the season, identifying immature lesions of FLS may indicate the leaves are no longer protected by a fungicide and may need to be treated again to finish the season, especially on highly susceptible varieties. Immature lesions on expanding leaves look similar to mature lesions; however, they lack the pronounced maroon or purple edge surrounding the lesion (Fig. 2) and have yet to sporulate. Even earlier, these immature lesions start out as faint water soaked spots, which can be seen alongside immature lesions on some leaves (Fig. 2). Alternately, immature lesions on fully expanded leaves often appear as unexpanded smaller spots or purple specks with faint tan center (Fig. 1). Given that strobilurin-resistant FLS has been detected in the state, (See earlier publications on this blog) immature lesions shortly after fungicide application in a strobilurin-alone (FRAC 11) fungicide program could indicate a fungicide-resistant strain is present in the field. Fungicides are not recommended on R6 soybeans. See MP 154 for fungicides labeled for use on soybean in Arkansas.
Sudden death syndrome has been observed on a few plants in some fields across the state. Symptoms for this disease often begin on one or a few plants in the field, commonly along the irrigation manifold where cool water favors disease development. Field-wide symptoms that look similar to SDS could indicate triazole phytotoxicity if recently (~ 14 d ago) sprayed. Triazole fungicides are often used to treat FLS, especially where strobilurin-resistant FLS has been detected or suspected in a field.
Finally, soybean rust (SBR) has not been observed in the state but was reported earlier this month in southern Louisiana (Fig. 3). In 2013, SBR was reported in mid-June at similar locations in southern LA. So, unless a tropical storm develops and distributes spores throughout the Mid-South, SBR will likely take a similar course as last year when it was first detected in August but did not spread statewide until October, too late to cause any significant yield loss. This was primarily due to dry conditions in southern Arkansas. It has been 10 years since SBR was first detected in the US. In that time there was only one year when the UA Extension Plant Pathologists recommended a fungicide to protect a major portion of Arkansas’ soybean crop. Since rust must be reintroduced each year into the state, early detection is crucial to management. An ongoing service provided to soybean growers in Arkansas is a network of plant pathologists in the southern states and Arkansas to detect initial infections and provide early warning of disease presence in the state. This service has played a key role in SBR management. Future updates will be posted on this blog site as SBR develops in the Mid-South.