Craig S. Rothrock

Cotton Diseases Incited by Rhizoctonia Solani

Cotton (Gossypium spp.) originated as a tropical/subtropical perennial plant, but is produced as an annual crop in many temperate regions around the world. The epicotyl of the cotton plant is poorly developed and thus the first true leaves may not expand for a period of several weeks. As a result of planting cotton under marginal environmental conditions (i.e., temperate regions) and the crops slow physiological development, the plant remains in the seedling stage for an extended period. Seedling diseases on cotton are a worldwide problem (Hillocks, 1992). An estimated 2.8% loss in production was associated with seedling diseases over the last ten years, 1986–1995, in the United States (Proceedings of the Beltwide Cotton Production Research Conferences, National Cotton Council). The primary pathogens in the seedling disease complex of cotton include Rhizoctonia solani, Pythium spp., Thielaviopsis basicola, and Fusarium spp. (Watkins, 1981; Ridgway et al., 1984). R. solani is a prevalent and important pathogen in the seedling disease complex of cotton wherever cotton is grown (Ray and McLauglin, 1942; Fulton and Bollenbacher, 1959; Ramey, 1962; Ford et al., 1981; Johnson et al., 1969, 1978; Davis, 1975; Smirnova, 1980; Roy and Bourland, 1982; Melero-Vara and Jimenez-Diaz, 1990; Colyer et al., 1991a; Lisker and Meiri, 1992; Moustafa-Mahmoud et al., 1993).

Evaluation of biological and chemical seed treatments to improve stand of snap bean across the southern United States

Thirteen bacterial, four fungal, and four chemical fungicide seed treatments were evaluated one or more years in multiple field locations across the southern United States. Snap bean seed was treated in bulk with fungicides and most biocontrol agents, shipped to individual locations, and stored until planting or treated on site immediately before planting. Populations of biocontrol agents on seeds were assayed after seed treatment and planting. Analysis of variance of percent plant stand at 28 days after sowing revealed highly significant (P<0.01) effects of location and treatment in 1996, 1997 and 1998. A treatment by location interaction also occurred in 1996 and 1997. When treatments tested in two or three years were analyzed together, no biological seed treatments significantly affected percent stand. Carboxin significantly increased percent stand compared with nontreated seed in data sets combined from 1997 and 1998 and 1996 to 1998; captan and carboxin plus metalaxyl also increased stand in 1997 and 1998. Improvements in efficacy and consistency of biological seed treatments are necessary before they can be recommended for use in snap bean production.

Efficacy of Seed Treatment Chemicals for Black Root Rot, Caused by Thielaviopsis basicola, on Cotton

The efficacy of triazole and host resistance-inducing seed treatment chemicals was examined for black root rot on cotton caused by Thielaviopsis basicola in both artificially and naturally infested soils with and without nematodes. In naturally infested soil, myclobutanil was effective in reducing root and hypocotyl discoloration over a wide range of soil population densities. Treatments containing high rates (42 g a.i./100 kg seed) of myclobutanil provided greater reductions in disease than low rates (21 g a.i./100 kg seed) in some experiments. Acibenzolar-S-methyl applied to the seed reduced black root rot or colonization by T. basicola on seedlings in artificially infested soils. Rates of acibenzolar-S-methyl did not differ in efficacy. In controlled studies, root colonization by T. basicola was significantly lower when seeds were treated with both myclobutanil and acibenzolar-S-methyl than with either chemical alone. In naturally infested soil under low (24 CFU/g soil) and high (154 CFU/g soil) populations of T. basicola, a combination of myclobutanil and acibenzolar-S-methyl at the high rate resulted in the lowest root discoloration and colonization. The nematicide seed treatment abamectin improved the control of black root rot in the presence of Meloidogyne incognita. The semi-selective medium TB-CEN allowed the importance of T. basicola to be evaluated in the presence of other pathogens that contribute to the seedling disease complex on cotton by quantifying the isolation frequency and percent colonization of T. basicola.

Using marked strains to assess the effect of hairy vetch amendment on the inoculum densities of Thielaviopsis basicola, Pythium ultimum and Rhizoctonia solani

Controlled environmental studies were performed to assess the survival and activity of Thielaviopsis basicola in hairy vetch-amended and non-amended soils by using benomyl tolerance as a biochemical marker. Benomyl-tolerant T. basicola populations in infested soil were assayed by dilution plating on benomyl-amended T. basicola–carrot–etridiazol–nystatin (TB–CEN), a selective medium for T. basicola. Both soil populations and isolation frequency of the benomyl-tolerant T. basicola strain from cotton seedlings were reduced in soils amended with hairy vetch. Disease severity was also lower in seedlings grown in hairy vetch-amended than in non-amended soils. Results were consistent with results obtained from the direct observation of T. basicola chlamydospores, indicating a loss in chlamydospore viability in hairy vetch-amended soils. The effects of hairy vetch amendment on Pythium ultimum and Rhizoctonia solani populations were assessed by using a kanamycin-tolerant P. ultimum strain and an AG-11 isolate of R. solani. Populations of the kanamycin-tolerant P. ultimum strain and R. solani AG-11 did not change significantly with hairy vetch amendments, but there was a trend toward larger populations, indicating that these fungi were not suppressed in amended soils. The results agree with field studies where populations of T. basicola, its isolation frequency from diseased cotton seedlings and the seedling disease severity caused by T. basicola were reduced in hairy vetch-amended soils, but not those of Pythium spp and R. solani. The use of marked strains of T. basicola may be useful in identifying agricultural practices that are potentially suppressive to this pathogen.

Effect of Fungicide Seed Treatments, Cultivars, and Soils on Soybean Stand Establishment

Seedling diseases are one of the major production problems for soybean. The primary control of soybean seedling diseases is by fungicide seed treatments but comparisons of seed treatments are difficult because stand responses are often erratic in the field. The objective of this study was to determine the efficacy of soybean fungicide seed treatments in naturally infested soils under controlled environments at three temperatures. Seed of three soybean cultivars were treated with six fungicide treatments or not treated and planted in two soil types collected from two fields in April, May, and June 2008 and 2009. Tests were conducted in growth chambers at 21°C (April planting), 25°C (May planting), or 28°C (June planting). Stands were determined when at least 25% of the seedlings reached the V4 growth stage. Seed treatments improved stands at all three temperatures, in both soils, and in both years. In general, the broad-spectrum fungicides trifloxystrobin + metalaxyl and mefenoxam + fludioxonil + azoxystrobin resulted in the highest stands. The selective fungicide treatments metalaxyl and pentachloronitrobenzene + carboxin were effective at all three temperatures, implying that Pythium spp. and Rhizoctonia solani, respectively, were involved in seedling disease and active over a range of temperatures. The efficacy of fungicides was not consistently associated with the cultivars used in this study.

Soil Texture Influence on Meloidogyne incognita and Thielaviopsis basicola and Their Interaction on Cotton

Microplots were used to evaluate the impact of soil texture on Meloidogyne incognita, Thielaviopsis basicola, and their interaction on cotton. A native silt loam soil (48% sand) and four different artificial soil textures produced by mixing native soil with sand (53, 70, 74, and 87% sand) were studied. Each soil texture was infested with 0, 4, or 8 M. incognita eggs and 0 or 20 T. basicola chlamydospore chains per gram of soil in a factorial treatment arrangement. Plots were watered when soil moisture fell below -10 joules/kg for the first 21 days and -30 joules/kg from 22 days to harvest. Plant growth was suppressed early in the season and midseason by T. basicola. M. incognita suppressed plant growth and delayed plant development late in the season across all soil textures. Cotton yield was lower in the presence of either T. basicola or M. incognita. An interaction between M. incognita and T. basicola, which decreased plant growth and yield, occurred in 2006 when neither pathogen caused substantial plant damage. Plant growth, development, and yield were lowest in soils with >74% sand. Root colonization by T. basicola and fungal reproduction and survival decreased in soil having 87% sand. M. incognita generally caused more galling and reproduction in soils as sand content increased. Root galling severity and M. incognita reproduction were suppressed by the presence of T. basicola in soil at sand contents lower than 87%. Soil texture had a greater impact on T. basicola than on M. incognita in this study.

Effect of root infection by Macrophomina phaseolina on stomatal conductance, canopy temperature and yield of soybean

Abstract Charcoal rot, caused by Macrophomina phaseolina, is a widespread drought-associated, soilborne disease of soybean. Generally, disease severity is assessed indirectly as host colonization at the end of the growing season. Few studies have measured the effects of disease during the season. Two moderately charcoal rot resistant cultivars (‘DT 97-4290‘ and ‘Delta Pineland 4546‘), a drought tolerant cultivar (‘R01-1581F’) and a susceptible cultivar (‘LS 980358‘) were planted in microplots filled with uninoculated steam pasteurized soil or pasteurized soil infested with M. phaseolina. Half of the plots were irrigated and half were drought stressed. Stomatal conductance and infrared canopy temperatures were measured 47, 70, 77 and 98 days after planting (DAP) in 2008 and 52, 63, 72 and 84 DAP in 2009. Yields were taken at the end of the season. There were significant differences in stomatal conductance. In 2008, stomatal conductance was significantly lower 47 DAP in the non-irrigated/inoculated plots than the other treatments. At 98 DAP, stomatal conductance was highest in the irrigated/non-inoculated plots. In 2009, stomatal conductance was significantly higher in the non-infested plots than the infested plots of ‘LS 890358‘ and ‘R01-1581F’ across sample times, but not in plots of the resistant cultivars ‘DT 97-4290‘ or ‘DPL 4546‘. These differences were reflected in significantly greater canopy temperatures and lower yields in the infested compared with the non-infested plots of the susceptible, but not the resistant cultivars. These results show that root infection by M. phaseolina can reduce water movement in soybean and reduce yields in susceptible cultivars.

Effects of Meloidogyne incognita and Thielaviopsis basicola on Cotton Growth and Root Morphology

Effects of the root-knot nematode Meloidogyne incognita and the fungal pathogen Thielaviopsis basicola on cotton seedling growth and root morphology were evaluated in controlled environmental experiments. Four pathogen treatments, including noninfested soil, soil infested with M. incognita, soil infested with T. basicola, and soil infested with both pathogens were evaluated at soil bulk densities (BDs) of 1.25 and 1.50 g/cm(3). Plant growth and the morphology of the root systems were evaluated 44 days after planting. Infestation with M. incognita and T. basicola together significantly reduced seedling emergence, number of stem nodes, and root system volume compared with either pathogen alone. Either M. incognita or T. basicola reduced plant height, root fresh weight, top dry weight; root parameters total root length, surface area, and links; and root topological parameters magnitude, altitude, and exterior path length. M. incognita infection increased root radius. Root colonization by T. basicola increased with the presence of M. incognita at the lower soil BD. In contrast to previous research with Pythium spp., root topological indices (TIs) were similar with all of the treatments. Root TIs were near 1.92, indicating a herringbone (less branching) root architectural structure. Studying root architecture using a topological model offers an additional approach to evaluating fungi and nematodes and their interactions for soilborne-pathogen systems.