by Rod Swoboda
In early summer, seedling diseases begin diminishing and foliar and root diseases are starting to show up. Types of diseases and the extent of infestation you see in your soybean fields in the summer depend a lot on the weather, explains X.B. Yang, an Iowa State University extension plant pathologist.
He provides the following discussion of some common soybean diseases (excluding viral diseases) you may see while you are scouting fields in July.
Phytophthora Root Rot
Already this year, the Iowa State University Plant Disease Clinic has received many soybean samples infected with Phytophthora. Last year, most infected samples were early seedling-stage plants (before the first true leaves), whereas most of the samples this season are plants above the V2 growth stage.
Calls from agronomists at seed companies indicate prevalence and damage by Phytophthora root rot, with some fields replanted twice. According to Brian Frischmeyer (SCE, Sully, IA), the disease is worse than last year with many fields having diseased plants. In a field of 45 acres, plants in almost the entire field were infected. Differences between soybean treated and untreated with chemicals were evident by the need for replanting in his area.
“Damping-off has occurred in some popular soybean varieties that have the Rps-1k gene,” he says. “This situation was anticipated because recent data indicated a drastic increase in Phytophthora races that can defeat the Rps-1k gene.”
He advises farmers and crop scouts to “take good notes on Phytophthora root rot location and level of infection in your fields, especially on varieties with the Rps-1k gene. Such information will help in your future management of this disease.”
Fungal Root Rot
Both Rhizoctonia root rot and Fusarium root rot have been found from many soybean fields, says Yang. Typical symptoms of these diseases are lack of lateral roots with discoloration (dark to red brown) on taproots.
Generally, the disease samples came from fields that also had other problems, mainly iron chlorosis in high-pH fields or herbicide stress. Application of some herbicides for weed control may stress soybean plants and thereby increase their susceptibility to fungal infection. If so, consider cultivation to promote root growth, which helps soybean grow out of the problem.
Bacterial blight has been found in only a few cases this year on soybeans. Normally, this disease occurs in Iowa every year without causing significant yield losses, says Yang. It is caused by the bacterium Pseudomonas syringae.
Lesions (small, angular, water-soaked, yellow-to-brown spots) of bacterial blight are normally first observed on top leaves. The lesions enlarge in rainy weather and merge to produce irregular dead areas.
Sometimes, brown spot can be mistaken for bacterial blight but the two diseases are easy to separate. Bacterial blight occurs on upper new leaves and brown spot infects aged leaves or leaves on the lower portion of plants. In parts of Iowa where there has been frequent rain, more bacterial blight may be observed.
A foliar disease commonly seen this month is brown spot, caused by the fungus Septoria glycines. Like bacterial blight, this disease occurs every year.
Disease symptoms occur on the lower leaves of soybean plants, explains Yang. The fungus spreads by splashing rain. Thus, warm-weather conditions may arrest the development of this disease. Symptoms include many irregular, dark brown spots on both upper and lower leaf surfaces. Adjacent lesions frequently merge to form irregularly shaped blotches.
“Brown spot usually does not cause damage unless the disease progresses due to frequent rains later in the season and there is considerable premature defoliation,” says Yang.