Suppression of Soilborne Diseases of Soybean With Cover Crops.

Field trials were conducted from 2010 to 2013 at four locations in Illinois to evaluate the impact of cover crops (cereal rye [Secale cereal], brown mustard [Brassica juncea], winter canola [B. napus], and winter rapeseed [B. napus]) on soybean [Glycine max] stands and yield, diseases, pathogen populations, and soil microbial communities. Cover crops were established in the fall each year and terminated the following spring either by using an herbicide (no-till farms), by incorporation (organic farm), or by an herbicide followed by incorporation (research farm). Although shifts in soilborne pathogen populations and microbial community structure were not detected, cover crops were found to induce general soil suppressiveness in some circumstances. Cereal rye and rapeseed improved soybean stands in plots inoculated with Rhizoctonia solani and decreased levels of soybean cyst nematode in the soil. Cereal rye increased soil suppressiveness to R. solani and Fusarium virguliforme, as measured in greenhouse bioassays. Cereal rye significantly improved yield when Rhizoctonia root rot was a problem.

Solanaceous Weeds as Possible Sources of Cucumber mosaic virus in Southern Illinois for Aphid Transmission to Pepper.

Over 5,000 individual plants representing approximately 55 species from an area in southern Illinois where Cucumber mosaic virus (CMV) has been a major problem in pepper (Capsicum annuum) were tested for the presence of CMV by enzyme-linked immunosorbent assay (ELISA). Representative ELISA-positive samples were checked by western blot tests to confirm virus-specific reactions. Nearly all of the infected plants detected were either Solanum ptycanthum (eastern black nightshade) or Physalis spp. (principally P. heterophylla, groundcherry). Over 1,000 pepper transplants and approximately 500 tomato transplants, collected prior to planting, were negative for CMV by ELISA. In aphid transmission (arena) experiments, all five aphid species tested were capable of transmitting CMV from nightshade to pepper: Aphis fabae subsp. solanella, Aphis gossypii, Myzus persicae, Rhopalosiphum padi, and Sitobion avenae. Aphis fabae subsp. solanella, A. gossypii, and A. nerii were able to transmit CMV from P. heterophylla to pepper. Aphis fabae subsp. solanella was commonly found colonizing nightshade from May through October in southern Illinois.

Influence of Inoculum Density of Verticillium dahliae on Root Discoloration of Horseradish.

The relationship between inoculum density (number of microsclerotia per gram of air-dried soil) of Verticillium dahliae at the time of planting and the severity and incidence of root discoloration of horseradish at harvest was investigated in a 2-year study conducted in the greenhouse, microplots, and commercial production fields. The objective of the study was to develop a disease-forecast system that would assist growers in assessing the risk of the disease before planting horseradish in a particular field. Significant correlations were observed between inoculum density and severity and incidence of root discoloration in the greenhouse and microplots, although the form of the relationship varied with trials from linear to quadratic and negative exponential. No correlation was found between inoculum density of V. dahliae and severity and incidence of root discoloration in commercial production fields. In some fields with low inoculum densities, high ratings of severity and incidence of root discoloration were observed even with the partially resistant cultivar 769A. Conversely, in other fields with high inoculum densities, low ratings of severity and incidence of discolored roots were observed even with the susceptible cultivar 647A. These results suggest that a disease-forecast system based solely on inoculum densities of V. dahliae would be unreliable under field conditions when the other factors affecting the inoculum density-disease relationships cannot be controlled. Knowing the amount of initial inoculum may, however, save growers from planting horseradish in highly infested fields, but it would not guarantee a disease-free crop in fields with low levels of infestation.

Verticillium dahliae Resistance in Horseradish Germ Plasm from the University of Illinois Collection.

A total of 113 horseradish cultivars from the University of Illinois germ plasm collection at Urbana were evaluated for their reaction to Verticillium dahliae in the greenhouse following a root-dip inoculation. Root discoloration was rated 2 months after inoculation on a scale of 0 to 3 as follows: 0 = no symptoms; 1 = trace to less than 10% of the root cross-section with vascular discoloration; 2 = 10 to 50% of the root cross-section with vascular discoloration; and 3 = more than 50% of the root cross-section with vascular discoloration. The cultivars exhibited a large amount of variation in response to V. dahliae infection, with mean root discoloration ratings ranging from 0.2 to 2.6. The frequency distribution of responses of the 113 cultivars was normal, with a mean and a standard deviation of 1.2 and 0.4, respectively. Six cultivars, 635A, 1236A, 769A, 125A, 761A, and 28A, were identified as resistant to V. dahliae. The existence of resistance to V. dahliae in horseradish germ plasm from the University of Illinois collection is a great resource for the breeding of improved horseradish cultivars that will combine resistance to V. dahliae with other desirable characters.

Evaluation of Inoculation Methods for Screening Horseradish Cultivars for Resistance to Verticillium dahliae.

Four inoculation methods-colonized oatseed, root dip, infested soil, and set dip-were tested in the greenhouse for their effectiveness in identifying horseradish cultivars that are resistant to Verticillium wilt of horseradish. Examination of the inoculum density-disease relationships derived with each of these methods on susceptible (647A) and resistant (769A) cultivars showed that all were effective, though at varying degrees, in differentiating between susceptible and resistant reactions. Results were more consistent with the root dip method as it produced the largest least-squares means difference in wilt index between the two cultivars, the highest r 2, the lowest coefficient of variation, the shortest incubation periods, and the highest incidence of foliar symptoms. Overall, inoculum concentrations accounted for only a small amount of the total variation in wilt index (0.14 ≤ r 2 ≤ 0.73). This observation, in accord with previous reports on other hosts of Verticillium dahliae, would suggest that inoculum densities may not be a good predictor of the severity of Verticillium wilt of horseradish.