The influence of environment and host growth for improved fungicide applications for control of southern stem rot of peanut
Rideout, Steven Lewis
MetadataShow full item record
Southern stem rot of peanut, caused by the soilborne fungus Sclerotium rolfsii Sacc., has long plagued peanut (Arachis hypogaea L.) producers in the United States. Management of this disease is obtained primarily with calendar-based fungicide applications. Although usually effective, the calendar-based program fails in some instances where environmental conditions favor earlier or later stem rot development than is normal. The overall objective of this research was to better understand the effects of environment on peanut stem rot development and the factors associated with chemical control. In an effort to better define the relationships between environmental conditions and stem rot development, disease progress and microclimatic conditions were monitored at four locations for three years. Disease progress curves from these 12 trials revealed considerable variation in stem rot development across growing seasons, however, consistency in disease progression across locations was noted within a given season. A consistent, significant (P < 0.05) correlation (positive) between stem rot incidence and canopy relative humidity was found for the three seasons. An algorithm, based upon soil temperature and canopy relative humidity, also provided a consistent, significant (P < 0.05) correlation (positive) with stem rot incidence. Protection intervals for currently registered peanut fungicides were also explored in this research. A detached branch method was developed in trials examining the effect of plant age on southern stem rot development. In growth chamber trials, younger plants were more susceptible to inoculations with S. rolfsii. Coinciding with the growth chamber trials, branches detached from younger plants were also more susceptible to S. rolfsii. The detached branch method was then successfully employed to determine the residual activity of four fungicides, often providing more consistent findings than data from field trials. Thifluzamide had the longest residual activity on S. rolfsii, followed by flutolanil and azoxystrobin, and tebuconazole. In-field disease assessments for southern stem rot are difficult due to the wide array of symptoms that can be expressed by infected peanut plants. Alternate assessment methods were constructed and tested against the current standard assessment method that quantifies disease incidence (percentage of infected 30.5-cm row segments). Stem rot assessments made immediately following peanut inversion correlated substantially better with pod yields than did assessments taken within the growing season. Of the tested assessment methods, the standard disease incidence method at harvest correlated well with yield and was most time efficient. In additional trials, effects of an in-furrow treatment of azoxystrobin were examined. The in-furrow treatment of azoxystrobin had minimal impact on plant stand counts, tomato spotted wilt incidence, yield, and crop value. However, in-furrow applications of azoxystrobin did suppress levels of Aspergillus crown rot (Aspergillus niger) and early season (prior to 60 days after planting) southern stem rot. However, season-long control of stem rot was only accomplished through the use of mid-season applications of azoxystrobin. Similarly, yields and crop value were only significantly increased when in-season applications of azoxystrobin were made. This study expands our understanding of southern stem rot epidemics, but also demonstrates the complexity of a pathogen that can cause disease equally well above or belowground depending on environmental conditions.