Verticillium Wilt on Resistant Tomato Cultivars in California: Virulence of Isolates from Plants and Soil and Relationship of Inoculum Density to Disease Incidence

GROGAN, R. G. , N. IOANNOU, R. W. SCHNEIDER, M. A. SALL, and K. A. KIMBLE. 1979. Verticillium wilt on resistant tomato cultivars in California: Virulence of isolates from plants and soil and relationship of inoculum density to disease incidence. Phytopathology 69:1176-1180. Verticillium wilt caused by race 2 of Verticillium dahliae is common in proportions) was about 5.7 ms per gram of, soil. In five other fields, California on tomato cultivars with the Ve gene for resistance to race 1. however, in which the numbers of race 2 ms were determined, a linear About 47% of 124 isolates of V. dahliae taken directly from tomato field correlation was observed between numbers of race 2 ms (0.0 to 2.0 ms/g of 'soils were race 2; the remaining isolates were either race 1 (43%) or soil) and DI (0 to 100%) when data were plotted arithmetically after nonpathogenic on tomato (about 10%). In contrast, isolates from diseased conversion of DI %to loge (1/ 1 -DI) (slope = 2.Oand r =0.877). The line for tomato plants (race I resistant cultivars) from the same fields were the same data plotted on a logio-loge-logio scale had a slope of 1.0(or 1.57 predominantly race 2 (about 86% of 153 tested). Race I was more virulent if assumed to be nonlinear and transformed to logio before regression than race 2 on cultivars lacking the Ve gene for resistance (susceptible). analysis) instead of 0.66, as predicted by Baker et al (10) for abstract Average virulence of race 2 isolates was lower on cultivars with the Ve gene mathematical Model II. Thus, the models and equivalent interpretations than on susceptible cultivars. Incidence of Verticillium wilt (DI) on race I for slopes of lines in arithmetic, logio-logio and log0o-log,-0ogio plots appear resistant cultivars was essentially 100% in 46 fields where soil inoculum to be of questionable validity. density (ID) of total microsclerotia (ms) (race 1 and 2 in undetermined Additional key words: Lycopersicon esculentum. Wilt of tomato (Lycopersicon esculentum L.) caused by these isolations were maintained on potato-dextrose agar (PDA) Verticillium dahliae Kleb. (microsclerotial form of V. albo-atrum slants and tested in the greenhouse for pathogenicity on susceptible Reinke and Berth.) was reported from California in 1926 (22). In and race 1 resistant cultivars. 1951, Schaible et al (23) reported a high level of resistance to Soil sampling and assay. Soil samples were collected from Verticillium wilt in a small-fruited wild tomato from Peru. This processing-tomato fields planted to race 1 resistant cultivars in the resistance, conferred by a single dominant gene (Ve) was Sacramento and San Joaquin valleys of California. Soil collections incorporated into most tomato cultivars grown in California and were made near the end of the growing season (usually within the for several years was effective in preventing losses from Verticillium last month before harvest) from three random rows per field. Ten wilt. Alexander (1) reported a new race of V. dahliae in Ohio 25-cm deep soil cores were taken from each row (at 10-15 m pathogenic on resistant tomato cultivars. Subsequently, a similar intervals) with a 2.5-cm diameter soil tube, and the 30 cores from new race (race 2) was reported in several European countries (21) each field were bulked into a composite sample. The samples were and, more recently, in North Carolina (11). In California, isolated returned to the laboratory, air-dried for 15 days at 20-27 C, instances of infection by V. dahliae in race 1 resistant cultivars of pulverized in a revolving jar mill (13) for about 25 min in a cold tomato were reported in 1972 by Hall and Kimble (15). room, and assayed for microsclerotia (ms) of V. dahliae using a In this study, we determined the inoculum density (total modification (18,19) of the wet-sieving technique (4,16). Triplicate microsclerotia [ms]) of V. dahliae in field soils and the relationship 15-g samples from each composite sample were wet-sieved through of inoculum density (ID) to the incidence of Verticillium wilt (DI) 125and 38-Mm sieves, and the soil residues retained by the second on race 1 resistant cultivars. The populations of V. dahliae in five sieve were treated with 0.5% NaOC1 (2) and cultured in 15 plates other fields were analyzed quantitatively for races and strains with containing modified pectate agar (18,19). After incubation for 12 different virulence on resistant and susceptible cultivars to days at 22-24 C, the soil was washed from the surface of the determine the relationship between virulence and ID of race 2 ms in medium and the microsclerotial colonies were counted with a soil and disease incidence. dissecting microscope. The number of propagules per gram of soil was calculated by assuming that each microsclerotial colony MATERIALS AND METHODS originated from a single propagule. To obtain cultures, agar blocks were taken from selected microsclerotial colonies and ms were Field surveys. In 1973 an extensive field survey of the major areas separated from the medium by grinding the agar block with a pestle of California producing processing tomato was undertaken to and mortar. The sucrose flotation technique of Huisman and determine the extent of Verticillium wilt caused by race 2 of V. Ashworth (17) also was used occasionally to isolate ms directly dahliae on race I resistant cultivars. Fields in which the disease was from soil samples. The separated ms were suspended in sterile found were recorded, but no attempt was made to determine the distilled water and collected on a -Am Millipore filter by vacuum percentage of diseased plants in each field. The disease was detected filtration. The ms then were treated with 0.5% NaOC1 for 5 sec, by examination of plants for foliar symptoms and vascular rinsed with sterile distilled water, and transferred individually into discoloration of the stem and by isolation of the pathogen from culture plates containing water agar (WA). Single spores from samples of discolored stems. Cultures of V. dahliae obtained from selected colonies were transferred onto PDA slants. Determination of disease incidence. The incidence of 00031-949X/79/000209$03.00/0 Verticillium wilt in tomato fields was determined at the same time @1979 The American Phytopathological Society that soil samples were collected for determination of ID. Three