Over the past decade, occurrence of potato southern blight caused by Sclerotium rolfsii Sacc. has increased on the Eastern Shore of Virginia (ESVA) and surrounding areas. There is no known potato cultivar resistant to S. rolfsii, and traditional management relying on fumigants and fungicides has provided inconsistent results. This research evaluated the effect of planting date and cultivar as strategies for reducing the impacts of potato southern blight. A factorial arrangement of four planting dates and ten cultivars were evaluated in field trials established in 2017. Artificial field inoculations were performed between 56 and 73 days post-planting, utilizing a composite inoculum of two S. rolfsii strains isolated from ESVA potato southern blight outbreaks in 2016. Preliminary results showed that the two earliest plantings (Mar 7 and 24) resulted in lower in-field disease incidence. Within those planting dates, the cultivars ‘Accumulator’, ‘Yukon Gold’, ‘Russet Burbank’, and ‘Atlantic’ provided the least incidence (11-22%). For the subsequent planting dates (Apr 14 and May 10), incidence increased, but ‘Accumulator’ significantly produced the least incidence in the last two planting dates (66-71%) compared to the other nine cultivars (89-100%). Additionally, the first two planting dates yielded the most marketable tubers (77-96%), compared to later planting dates (9-71%). Among cultivars, ‘Accumulator’ presented more tolerance to infection until the third and fourth planting date. In general, southern blight significantly decreased yield and tuber quality in later planting dates; therefore, early planting dates coupled with tolerant cultivars are recommended as strategies to suppress potato southern blight.
The main goal was to identify potential cultural control options in managing southern blight to ensure the sustainability of potato production in regions impacted by this disease through the following objectives:
- Evaluate the effect of planting dates on southern blight incidence in potato, by establishing field trials examining four different planting dates and assessing disease incidence, yield parameters, and tuber quality.
- Evaluate the effect of cultivar and the interaction with planting dates on the incidence of southern blight, by initiating field trials with ten popular cultivars and assessing disease incidence, yield parameters, and tuber quality.
- Determine the effect of planting dates and cultivars on the susceptibility of post-harvest infection by S. rolfsii, through the evaluation of the potato tuber susceptibility.
A field trial of 0.60 acres was established in 2017 at Virginia Tech’s Eastern Shore Agricultural Research and Extension Center (ESAREC) in Painter, VA, using a factorial arrangement of treatments in a split-plot design with four replications. Each main plot was 25 ft long x 60 ft wide with a single guard row on each side and 20 ft wide buffers between blocks; each sub-plot is 6 ft wide (2 rows) x 25 ft long. The seed was hand planted 10 in apart within rows with a 36 in row spacing.
Two factors were examined: planting date and cultivar. The four planting dates and ten cultivars combine to form a total of 40 treatments. The 2017 planting dates (Mar 7, Mar 24, Apr 14, and May 10) were chosen because they are within the normal recommended date range for commercial production on the ESVA and surrounding areas.
The ten cultivars included the most commercially popular cultivars in the mid-Atlantic region, including the ESVA (Richardson 2016). These contained early, middle, and late season cultivars. In addition, different potato types are represented (russet, white, red, gold, and specialty). This allowed us to evaluate the cultivar behavior after infection with S. rolfsii, the interactions between planting dates and cultivars, and yield and quality.
Field inoculation with two virulent strains of S. rolfsii was carried out between 56 and 73 days post-planting. The field inoculation was performed by adapting the Shokes et al. (1996) technique; replacing oat with millet seed.
The stand count was enumerated weekly until the stem and leaves had fully developed in the majority of plants. Disease incidence (percentage of plants with disease symptoms per plot) was evaluated during the growing season. Diseased plants were determined by visual inspection of symptoms and the presence of mycelium and/or sclerotia on the base of the stem.
Plants were harvested at the average number of days recommended for the ten cultivars (̴ 105 days on average). Each plot was machine lifted, and potato tubers from each plot were collected separately in bins. Furthermore, tubers were weighed and graded according to USDA standards. Tubers were inspected visually, and symptomatic tubers with brown, yellow, or tan round sunken lesions, white mycelium, soft tissue, or odorless cheesy material, were considered diseased (Weber 1943).
Utilizing JMP® Pro 13, means comparisons were conducted by analysis of variance and separated using Fisher Least Significant Difference test (LSD) at 5% significance level.
In-field Southern Blight Incidence Prior to Harvest
Effect of Planting Date in Marketable Yield
Based on the preliminary results, the reaction of the cultivars to southern blight varied between planting date. As the planting date was delayed, the incidence of southern blight was more severe. Within each planting date, some cultivars showed a greater tolerance. For example, in the Mar 7 planting date, ‘Atlantic’, ‘Russet Burbank’, ‘Yukon Gold’, and ‘Accumulator’ presented the least incidence statistically, compared with other cultivars, and this trend was maintained for the second planting date. In the Apr 14 and May 10 planting dates, ‘Accumulator’ significantly presented the least incidence.
Regarding the yield effect of planting date in fields inoculated with S. rolfsii, measurements of marketable tubers showed that the Mar 7 and Mar 24 planting dates generated greater yield. This yield was substantially reduced for the last two plantings. ‘Accumulator’ produced greater productivity in the first three plantings compared to the other nine cultivars. In comparison with the other cultivars in the third planting date, ‘Accumulator’ produced the highest yield (17,000 lb/A). In the May 10 planting date, all the cultivars had substantially poor yields, less than commercially acceptable.
The preliminary results show that the field incidence of southern blight varied markedly between planting dates and cultivars. Later planting dates significantly decreased potato yields. Therefore, early planting dates coupled with tolerant cultivars are recommended as strategies to suppress potato southern blight.
- Analysis of laboratory results will be performed
- A replicate of the project will be conducted during the 2018 growing season.
- The project results will be presented at the 2018 Plant Pathologist of the Future at the 2018 APS Pacific Division Meeting, and a poster session at the 2018 International Congress of Plant Pathology in Boston, MA. Additionally, local presentations to growers and agricultural professionals will be made in 2018.
- Richardson, B. 2016. 2016-Virginia Potato & Vegetable Review. VDACS:29. Available at: http://www.vdacs.virginia.gov/pdf/pvr.pdf (Accessed July 24, 2017).
- Shokes, F., Róźalski, K., Gorbet, D., Brenneman, T., and Berger, D. 1996. Techniques for Inoculation of Peanut with Sclerotium rolfsii in the Greenhouse and Field 1. Peanut Science 23:124-128.
- Weber, G. F. 1943. Southern blight, Corticium rolfsii of potato tubers. Phytopathology 33:615-617.
Educational & Outreach Activities
During 2017, the field trial was included in the plant disease identification and management demonstrations for the 4-H youth and graduate students’ tours that were held at the ESAREC in Painter, VA.
The research was presented to potato growers and agricultural professionals at the 2018 Eastern Shore Agricultural Conference & Trade Show in Melfa, VA, on January 24, 2018. Later, the research was presented at the 73rd meeting of the NJDelMarVaPa Extension Plant Pathologists Meeting in Newark, DE. And finally, the preliminary results of the project were presented at the 2018 Potomac Division 74th Annual Meeting in Ocean City, MD, on March 22, 2018.
The project will generate information that is not currently available. This information will provide options for farmers to select potato cultivars and planting date to minimize the negative impact on yield caused by southern blight, as well as cultivar selection options for breeders. Likewise, this could reduce the use of pesticides as a primary option for the management of southern blight.
Because of this project, we have been able to understand the necessity of this information for farmers. Working with farmers and presenting results to them, it has become clear that they value this information for making crop production decisions.