Location: Crop Genetics Research2013 Annual Report
1a. Objectives (from AD-416):
The objective to evaluate the resistance of public and private soybean varieties and breeding lines to the most serious diseases present in the southern United States.
1b. Approach (from AD-416):
The Delta Research and Extension Center, Mississippi Agricultural and Forestry Experiment Station, Stoneville, Mississippi, will obtain seed of the entries in the Mississippi State Soybean Variety Trials, which average 225 entries in soybean Maturity Groups III-VII. Seed of the breeding lines from the USDA soybean breeder at Stoneville will also be obtained. These entries and breeding lines will be screened for resistance to the following diseases: 1. STEM CANKER - The toothpick method of inoculation will be used to evaluate resistance to this disease. 2. FROGEYE LEAF SPOT - Spores from selected races which are prevalent in Mississippi will be sprayed on soybeans to be evaluated in mist chambers and in field studies. 3. CERCOSPORA BLIGHT, PURPLE LEAF STAIN AND POD ROT - The leaves of the entries will be inoculated with spores in mist chamber and field studies. Mature green pods will be placed in humidity chambers and inoculated with spores of the fungus. 4. BLACK ROOT ROT - Seed of the entries will be planted in potting mix infested with the fungus. Root ratings will be made 6 weeks after planting. Other foliar diseases of soybean will be evaluated if present in field plots.
3. Progress Report:
Seed were obtained from all of the entries in the Mississippi State Soybean Variety Trials. This included seed of Maturity Groups IV to VI and both Roundup Ready and conventional seed. Greenhouse, growth chamber, and field trials were conducted to determine the resistance of the entries to certain diseases. All of the entries in the 2012 Mississippi State Variety trials were evaluated for stem canker resistance. The results were published in Plant Disease Management Reports. The 2012 growing season was abnormally hot throughout the season. Therefore, stem canker may not have had time to develop in the Maturity Group IV entries. There are still several entries in the trials which are susceptible to stem canker. Any entry rated above a 2.0 has the potential to have several yield losses due to stem canker. All of the entries in the 2012 trials have been evaluated for frogeye leaf spot in greenhouse and field studies. The results were published in Plant Disease Management Reports. Entries from every maturity group were susceptible to frogeye leaf spot. Even though a large number of frogeye susceptible varieties are planted, losses to frogeye leaf spot have been minimal in recent years. This is probably because of the widespread use of foliar applied strobulurin fungicides. With the emergence of strobulurin resistant frogeye cultures, the incidence of this disease is sure to increase. The entries in the 2012 trials were inoculated with the purple leaf stain fungus at the R5 growth stage. The results were published in Plant Disease Management Reports. Many entries were susceptible to this disease. This is usually the foliar disease which causes the largest yield losses in Mississippi. Foliar applied fungicides are erratic in the control of this disease. Black root rot is becoming a problem in areas where soybeans are grown in soil with a history of being cropped to cotton. This fungus also causes black root rot of cotton. Cool wet conditions are needed early in the growing season for this disease to become a problem. The entries in the 2012 trials were inoculated with the black root rot fungus in the field. The disease failed to develop, therefore, we are in the process of evaluating the entries in growth chamber studies. We should finish our evaluations this summer. A trial was planted late in 2012 to evaluate soybean rust resistance. However, no rust developed in the trials. Seed were collected from a foliar fungicide trials and plated out to determine the incidence of purple leaf stain and Phomophsis on the seed. There were significant differences in incidence of both purple leaf stain and Phomophsis in some of the trials. Seed treatments were evaluated in plots inoculated with Rhizoctonia solani or Pythium species for percent seedling survival and soybean yield. Several of the seed treatments significantly increase yields over the untreated seed. Purple pod rot and Phomophsis seed decay diseases were evaluated by collecting mature green soybean pods of each variety from the field, surface disinfecting them, and placing them in humidity chambers. The pods were inoculated with spores of Cercospora kikuchii (purple pod rot) or Phomophsis longicola (Phomopsis seed decay) and incubated for two weeks. There were significant differences in purple pod rot ratings. We have isolated several cultures of this fungus and are testing them for virulence. Phomophsis seed decay incidence was high. Most of the varieties were susceptible to this disease. However, there was a range of reactions to this disease and some entries had more resistance than others. The black root rot trials conducted in the growth chamber are ongoing. Most of the entries were susceptible to black root rot. This disease may become a major problem as more soybeans are grown on land previously cropped to cotton.