Verticillium Wilt Disease of Tomato: Influence of Inoculum Density and Root Extension Upon Disease Severity

ASHWORTH, L. J., Jr., 0. C. HUISMAN, D. M. HARPER, and L. K. STROMBERG. 1979. Verticillium wilt disease of tomato: Influence of inoculum density and root extension upon disease severity. Phytopathology 69:490-492. Rate of infection of tomato by Verticillium dahliae increased with and plant growth otherwise was favored. Maximum loss under these increases of inoculum density in four field experiments in which conditions was about 40%, depending upon the experiment. In a sandy microsclerotia (MS) in naturally infected tomato plants was the inoculum. loam soil with a dense layer 15-60 cm below the surface however, the However, essentially all plants were infected by the end of the growing combination of wilt disease, restricted root growth, and poor water season regardless of inoculum density (range = 0.1-27 MS/g soil). Only penetration was devastating to tomato. Maximum yield under these slight external symptoms appeared in plants grown at inoculum densities of conditions was about 4.3 X 10' kg/ha compared with about 14 X 10' kg/ha 0.1-9 MS/g in a friable clay loam soil in which root growth was unrestricted under the more favorable growing conditions. Additional key words: V. albo-atrum, soilborne pathogens, fungal disease, epidemiology. Verticillium wilt of tomato (Lycopersicon esculentum Mill.) has moisture tension increases (9). It limits root extension when it been viewed as an unimportant disease by some workers and as offers an impedance greater than about 18 bars (4). Strength of the extremely important by others, as discussed by Jones and Crill (6). dense layer at KHFS ranges up to 30 bars. In contrast, West Side The definitive tests of the latter, in which noninoculated and Field Station (WSFS) soil was homogeneously of low strength to a inoculated differentially tolerant cultivars were transplanted in the considerable depth and offered little impedance to root growth (4). field, confirmed the yield reducing potential of Verticillium dahliae We followed the procedure by Newman (7), as modified by Grimes, Kleb. , the cause of this wilt disease. Yield reductions were 67% for et al (4), to determine root length. Briefly, root segments are cultivar Bonny Best, 38% for Walter, and 2 and 6% respectively, washed from duplicate 50-g soil samples dispersed with 1% sodium for Tropic and Florida MH-1. Recognition in 1971 of a strain of V. hexametaphosphate. They are decanted into a 15-cm Biichnerdahliae that induced severe disease of cultivars having the Ve gene type funnel and the filter paper is removed and sandwiched between for resistance (8) caused renewed interest in the disease in Plexiglas sheets. One sheet was etched with 10 mm squares. Root California (5). The new strain is called tomato strain-2 (5); it affects segments crossing one or more lines in 20 random squares were cultivars with the Ve gene like earlier prevalent populations counted and those data formed the basis for calculating centimeters affected older cultivars such as New Improved Pearson and Early of roots per cubic centimeter of soil. Pak-7 (K. A. Kimble,personal communication). Jones and Crill (6) Tomato cultivar Early Pak-7 was used in all tests; it is intolerant demonstrated the yield reducing potential of V. dahliae to of the V. dahliae populations prevalent in California soils, inoculated tomato plants, but used only one inoculum density of a including the pathogenic type named Tomato strain-2 recently conidial/mycelial suspension in their tests. discovered by Hall and Kimble (5). Plots were planted in early To determine the rate of infection and disease severity for cotton April and mature fruit were hand harvested once in late August to as well as tomato in differentially infested blocks in the field, we late September. Sunburned fruit were counted separately in both used microsclerotia (MS) in naturally infected tomato plants as 1974 and 1975. inoculum. Results for the cotton tests are published separately (2). Other more general experimental procedures are described in the We also studied root density in relation to infection and disease preceding paper (2). severity. Tests were made to determine whether plants with restricted root systems were more severely affected by wilt disease RESULTS AND DISCUSSION than those with unrestricted root systems. We became interested in mapping root systems following a report by Grimes, et al (4), which Influence of soils on root development. Tomato plants grown in showed that root maps through depths of 150 cm accurately the high-strength soil at KHFS produced more roots in the surface described water extraction by cotton and corn. For instance, plants 30 cm than those grown in the low-strength soil at WSFS. Root with roots restricted by a compact subsurface layer, used little of lengths were similar at the 30-60 cm depth at both locations, but at the water in deeper layers and required more frequent irrigation greater depths, root length was always greater in the low-strength than those with unrestricted root systems. soil (Fig. 1). Perhaps of more importance, root lengths were similar through 90 cm in the low-strength soil whereas root lengths MATERIALS AND METHODS diminished at each 30 cm increment of depth in the high-strength Tests were made at two locations each in 1974 and 1975 but soil (Fig. 1). inoculum, except for small amounts of native inoculum, was the Inoculum density and infection. Amounts of infection generally were low through mid July, regardless of inoculum density. Data same at both locations (2). Soils were markedly different. Grimes et for the 1975 WSFS test (Fig. 2) are typical of all four tests. We al (4) described the Hanford sandy loam soil at the Kearney found that tomato, like cotton (2), had little infection (based upon Horticultural Field Station (KHFS) as "high strength" because it isolation results) during the first 3 mo of the growing season. had a compacted layer at depths varying from 15--60 cm below the Thereafter, rates of infection varied with inoculum density, with surface. Soil strength is a measure of bulk density and increases as differences between inoculum densities being greatest 1-1.5 mo 00031-949X/79/000089$03.00/0 before harvest. By harvest, however, essentially all plants were 01979 The American Phytopathological Society infected, regardless of inoculum density (Fig. 2). Tomato cultivar