D. E. Mathre

Combined biological and chemical seed treatments for control of two seedling diseases of Sh2 sweet corn

Experiments were conducted in both the glasshouse and in the field to determine if biological and chemical control agents could be used together on sweet corn (Zea mays L.) seed to control Pythium ultimum damping-off or Penicillium oxalicum seedling disease. Pseudomonas aureofaciens AB254 and Pseudomonas sp. AB842 were used for control of P. ultimum and P. oxalicum, respectively. Metalaxyl seed treatment for control of P. ultimum was used at rates from 100 to 0.01% of the recommended rate either alone or in combination with P. aureofaciens AB254. Imazalil seed treatment for control of P. oxalicum was used at rates from 100 to 1% of the recommended rate either alone or in combination with Pseudomonas sp. AB842. In field tests, combining a chemical treatment with the biological agent did not affect the efficacy of disease control. Nor did combining low rates of chemical with the biocontrol agent increase the efficacy or reliability of disease control.

Survival of Teliospores of Tilletia indica in Soil

This study was conducted to assess survival of Tilletia indica teliospores in a location in the northern United States. Soils differing in texture and other characteristics were collected from four locations, equilibrated to -0.3 MPa, and infested with teliospores of T. indica to give a density of 103 teliospores per gram of dry soil. Samples (22 g) of the infested soil were placed in 20-μm mesh polyester bags, which were sealed and placed at 2-, 10-, and 25-cm depths in polyvinyl chloride tubes containing the same field soil as the infested bags. Tubes were buried vertically in the ground at Bozeman, MT, in October 1997. Soil samples were assayed for recovery and germination of T. indica teliospores 1 day and 8, 20, and 32 months after incorporation of teliospores into soil. The rates of teliospores recovered from soil samples were 90.2, 18.7, 16.1, and 13.3% after 1 day and 8, 20, and 32 months after incorporation of teliospores into soil, respectively, and was significantly (P < 0.01) affected by soil source. The percentage of teliospore recovery from soil was the greatest in loam soil and lowest from a silt loam soil. The rate of teliospores recovered from soil was not significantly affected by depth of burial and the soil source-depth interaction during the 32-month period. The percentage of germination of teliospores was significantly (P < 0.01) affected by soil source and depth of burial over the 32-month period. The mean percentage of teliospore germination at 1 day, and 8, 20, and 32 months after incorporation into soils was 51.3, 15.1, 16.4, and 16.5%, respectively. In another experiment, samples of silty clay loam soil with 5 × 103 teliospores of T. indica per gram of soil were stored at different temperatures in the laboratory. After 37 months of incubation at 22, 4, -5, and -18°C, the rates of teliospore recovered from soil were 1.6, 2.0, 5.7, and 11.3%, respectively. The percentage of spore germination from soil samples was highest at -5°C. Microscopy studies revealed that disintegration of teliospores begin after breakdown of the sheath-covering teliospore. The results of this study showed that teliospores of T. indica can survive in Montana for more than 32 months and remain viable.

SOURCES OF RESISTANCE TO CEPHALOSPORIUM GRAMINEUM IN TRITICUM AND AGROPYRON SPECIES

SummaryResistance to the soil-borne pathogen Cephalosporium gramineum was evaluated in Agropyron elongatum, A. intermedium. A. intermedium var. trichophorum, an Agrotriticum, and eight species of Triticum. Only A. elongatum and A. intermedium showed high levels of resistance. Agrotriticum (56 chromosomes) was resistant too. High resistance to C. gramineum is available, but its utilization will probably require the use of chromosome substitution techniques to transfer the resistance into an agronomically useful wheat.

A Method for Extraction and Enumeration of Teliospores of Tilletia indica, T. controversa, and T. barclayana in Soil

A sucrose-centrifugation method was developed to extract teliospores of Tilletia indica, T. con-troversa, and T. barclayana from soil. Six soil types were artificially infested with teliospores of each of the three fungi separately to produce 102, 103, 104, or 105 teliospores per 10 g. Each 10 g of infested soil was suspended in 200 ml of water with one drop of Tween 20 and shaken for 30 s. The soil suspension was first passed through a 117-μm sieve and then through a 53-μm mesh filter, and the filtrate was collected. The filtrate was then passed through a 20-μm mesh filter, and materials caught on the mesh were washed into two 50-ml centrifuge tubes and spun for 3 min (1,200 × g). The pellet was suspended in 1.6 M sucrose solution and centrifuged for 40 s (200 × g). The supernatant was passed through a 20-μm mesh filter. The materials caught on the 20-μm mesh were collected, and the number of teliospores was determined. This procedure was initially used to extract teliospores of T. indica in soil. For extraction of teliospores of T. contro-versa and T. barclayana, 1.0 M and 1.3 M sucrose solutions, respectively, were used, and the 20-μm mesh was replaced with a 13-μm mesh filter. Teliospores of T. indica, T. controversa, T. barclayana, and T. indica-like fungus on rye grass were successfully extracted from naturally infested soils. The relationships between number of teliospores recovered from the soil and number of teliospores incorporated into the soil were Ŷ= -0.60 + 1.28(X) - 0.04(X2),Ŷ = -1.25 + 1.56(X) - 0.07(X 2), and Ŷ = -0.71 + 1.33( X) - 0.04(X2) for T. indica, T. controversa, and T. barclayana, respectively, where Ŷ = log10 of the number of teliospores recovered from soil and X = log10 of the actual number of teliospores in soil.

CHARACTERIZATION AND EFFECTIVENESS OF PHIALOPHORA SPP. ISOLATED FROM A MONTANA TAKE-ALL SUPPRESSIVE SOIL IN CONTROLLING TAKE-ALL DISEASE OF WHEAT

Selected fungi from a take-all suppressive wheat field in Montana were characterized morphologically and identified as Phialophora spp. Ribosomal DNA (rDNA) fragments from four Phialophora spp. and two known Gaeumannomyces isolates were amplified with polymerase chain reaction (PCR) using universal primers, cloned, and sequenced. Sequence comparison of the rDNA ITS regions revealed that these Phialophora isolates were not closely related to Gaeummanomyces sp. Alignment of the ITS sequences allowed the design of PCR primers that distinguished the Phialophora isolates from Gaeummanomyces sp. as well as Phialophora sp. related to Gaeummanomyces. Phialophora sp. 1-52 and Phialophora sp. 1-58 were tested alone and in combination for suppression of Gaeumannomyces graminis var. tritici in the field. When introduced on autoclaved canola seed, 1-52 proved to be an efficient biological control agent against wheat take-all disease in its original suppressive soil, as well as in a highly conducive soil. Isolate 1-58 ...

Influence of plant age, disease incubation time, and host plant resistance on Verticillium wilt symptom expression in lucerne (Medicago sativa)

SummaryThe most economical means to control Verticillium wilt in lucerne, caused by Verticillium albo-atrum, is through resistant cultivars. Most Verticillium wilt resistance evaluations use some form of root-cut-soak inoculation procedure. In Trial 1, three lucerne cultivars were inoculated at eight plant ages and plant symptoms were scored at ten successive weekly intervals. Trial 2 consisted of two cultivars inoculated at ten plant ages and symptoms were evaluated at eight weekly intervals. In an experiment to study the genetics of resistance to this pathogen, four plants showing no visual symptoms were selected from each of four germplasms. The selected populations were crossed in diallel fashion. The population crosses plus the parents were evaluated for resistance to V. albo-atrum. Results from Trial 1 and Trial 2 showed disease severity increased with increasing inoculation age and with increasing scoring age. Cultivar symptom expression interacted with inoculation age, but not with scoring age. The diallel analysis supported resistance to Verticillium wilt being conferred by additive gene action, as heterosis was not detected and general combining ability mean square was 3.75 times larger than the specific combining ability mean square.