Characterization and persistence of Verticillium albo‐atrum isolated from alfalfa growing in high temperature regions of southern California

Abstract

Verticillium albo‐atrum isolated from alfalfa (lucerne) (Medicago sativa) growing at high air temperatures in southern California desert regions was compared with isolates from cooler, more favourable climates in Europe and the northern USA by determination of colony morphology, temperature effects on radial growth on agar, virulence on a resistant (NK‐89786) and susceptible (Moapa‐69) alfalfa cultivar, and by isozyme electrophoresis. Isolates from Europe, northern USA and southern California produced different colony morphologies on lactose yeast prune agar. Southern California isolates incubated at 30°C on agar produced significantly (P = 005) greater colony diameters than isolates from northern USA and Europe after 20 days. The virulence of isolates from southern California and northern USA was equally severe on alfalfa, while the virulence of European isolates was comparatively less severe. Alfalfa isolates of V. albo‐atrum from southern California had similar isozyme banding patterns to alfalfa isolates from Europe and northern USA, suggesting a common origin. However, differences in morphology, virulence and temperature relations indicate that isolates may have adapted to local environmental conditions. V. albo‐atrum from southern California was tested for its persistence in alfalfa debris at high temperatures over time, and for its ability to infect other crops and weeds. The fungus persisted over a 32‐month period in baled alfalfa hay exposed to air temperatures as high as 40°C during the summer months. Alfalfa stem pieces infected with V. albo‐atrum were buried in sterile soil or in non‐sterile field soil, or held without soil and incubated at 20, 24, 30, 35 and 41°C. Regrowth continued for up to 16 months at 30°C and for 3 months at 35°C from a small percentage of stem pieces buried in field soil. Regrowth continued for up to 16 months at 41°C from a small percentage of stems stored without soil. V. albo‐atrum was reisolated from all nine inoculated weed species, with three of these exhibiting foliar symptoms of Verticillium wilt. Ten of the inoculated crop species harboured V. albo‐atrum and four showed symptoms.

Verticillium survives heat in Mojave Desert alfalfa

“…In September 1990 the percentage of isolation of V. albo-atrum increased to a range of about 85% to loo%, but at the 16-to-20-inch (40 to 50 cm) level the incidence was reduced to about 48%. Although V. albo-atrum grows best at 77°F (25°C) and does not grow at temperatures above 86°F (30°C) (Howell and Erwin 1995), these results indicate that the fungus can survive in the field at unfavorably high air temperatures.…”

“…V. albo-atrum survived for 32 months in dry baled alfalfa hay (Howell and Erwin 1995) collected from a naturally infested field at Newberry Springs and stored in Riverside at temperatures averaging 86°F (30°C); maximum was 104°F (40°C).…”

“…albo-atrum resembles V. dahliae, the cause of Verticillium wilt of cotton, but does not produce microsclerotia and hence does not survive as well in soil. The optimum temperature for growth of V. albo-atrum is 75" f 2°F (24" k 1"C), with only slight growth at 86°F (30°C) and no growth at 91°F (33°C) (Howell and Erwin 1995).…”

“…There are no reports of Verticillium wilt in forage or seed-producing fields in the central valleys (Sacramento and San Joaquin) or in the lower desert valleys (Imperial, Coachella and Palo Verde). However, one isolate of V. albo-atrum was obtained from a sample sent to us from the Palo Verde Valley (Howell and Erwin 1995). The valleys Although known in Europe for de-in which Verticillium wilt has not been found constitute about 85% of the acreage in alfalfa in California.…”

Scientific Opinion on the pest categorisation of Verticillium albo‐atrum sensu stricto Reinke and Berthold, V. alfalfae Inderb., HW Platt, RM Bostock, RM Davis & KV Subbarao, sp. nov., and V. nonalfalfae Inderb., HW Platt, RM Bostock, RM Davis & KV Subbarao, sp. nov.

“…The most important of the EU hosts of the pathogen (Table 10) V. albo-atrum sensu lato survives in the soil as thick-walled mycelium (resting mycelium) either associated with infected host plant debris or as free-resting mycelium, following the decomposition of the infected plant tissues (Pegg and Brady, 2002). The fungus has been shown to persist as resting mycelium in dry alfalfa hay in the soil for up to 32 months (Basu, 1987;Howell and Erwin, 1995). Debris of infected host plants may serve as a principal means of the pathogen's survival and can act as reservoir of inoculum, by supporting conidia production.…”