Survival of Fusarium oxysporum f. sp. vasinfectum Chlamydospores Under Solarization Temperatures

Bennett, R. S. 2012. Survival of Fusarium oxysporum f. sp. vasinfectum chlamydospores under solarization temperatures. Plant Dis. 96:1564-1568. Solarization is an effective soil treatment against race 4 of Fusarium oxysporum f. sp. vasinfectum. Despite the lack of effective alternatives, solarization is rarely used in cotton because of its high cost. Use of solarization might be increased if soil temperatures could be used to predict reductions in pathogen inoculum levels, thereby ensuring high levels of efficacy. However, relationships between survival of race 4 chlamydospores, soil moisture, and temperatures typical of solarized soil are not known. Survival of cultureand plant-produced chlamydospores of race 4, incubated at 40°C in dry or moist environments, indicated the importance of moisture in determining spore survival. Mortality of spores from either source was low under dry conditions and much higher under moist conditions. A 6-week exposure of culture-produced chlamydospores to temperatures from 30 to 40°C under moist conditions indicated limited mortality at temperatures ≤35°C. However, most spores were eliminated by day 6 at 40°C. A second study using a moist environment indicated high mortality of spores by 5 weeks at 37°C or by 10 days at temperatures from 39 to 41°C. These results should serve as useful guides in efforts to develop solarization protocols that maximize effectiveness based on monitored soil temperatures. Few options are currently available in California cotton (Gossypium spp.) for managing Fusarium wilt caused by race 4 of Fusarium oxysporum f. sp. vasinfectum W.C. Snyder & H.N. Hansen (7). Availability of commercial Pima (Gossypium barbadense L.) cultivars highly resistant to race 4 are presently limited to Phytogen 800 and 805RF (Dow AgroSciences) (12,31). These cultivars accounted for 72% of the 2011 Pima acreage in California (32) but neither is immune to race 4. Significant early-season stand losses have been observed for Phytogen 800 and 805RF in sites with high levels of inoculum (R. S. Bennett, unpublished data). Furthermore, high levels of resistance to race 4 are presently unavailable in upland cotton cultivars (G. hirsutum L.; 12,30), which represent 98% of U.S. cotton acreage (33). Strategies for managing populations of the root-knot nematode Meloidogyne incognita (Kofoid & White) Chitwood (13,18,26), which aid in managing Fusarium wilt caused by races 1 and 2, are not applicable to race 4 because race 4 causes severe disease in both clay loam and sandy soils in the absence of nematodes (3,17). Soil treatments for reducing pathogen inoculum are generally too costly for large-scale use in cotton but may be feasible for treating limited disease hotspots. The most effective fumigants for reducing Fusarium wilt in a recent study, 50:50 (wt/wt) methylbromide + chloropicrin, and 60:40 (vol/vol) chloropicrin + 1,3dichloropropene, cost more than $2,100/acre to apply (3). However, the same study found that 6 weeks of soil solarization (soil heating under transparent plastic film; 16) in July and August was as effective as fumigation for reducing soil counts of F. oxysporum and plant mortality. Advantages of soil solarization over fumigation include the lack of environmental and safety regulations required for application of solarization and reduced chance of creating a biological vacuum that enhances subsequent recolonization by pathogens (14). The reduction in disease from a single solarization application may also persist for multiple years (15). Unfortunately, solarization is also expensive (approximately $1,200/acre for polyethylene film) and the treated area must be idled for the duration of treatment. Furthermore, treatment of disease hotspots may pose logistical challenges associated with moving plasticlaying equipment and water to remote sites within fields under production. Once in place, the treated area may obstruct the movement of irrigation water to the surrounding crop. Despite these limitations, solarization may be a viable management tactic if a high level of efficacy could be guaranteed. The efficacy of solarization might be optimized by monitoring soil temperatures and adjusting duration of treatment to maximize inoculum reduction. Use of soil temperatures to predict inoculum reduction requires detailed knowledge of the ability of F. oxysporum f. sp. vasinfectum chlamydospores to survive under solarization conditions. In the Central Valley of California, temperatures of approximately 34 to 38°C at a depth of 30 cm can be attained during the period from June through August in solarized plots (5; R. Bennett, unpublished data). Several studies have measured the effects of temperatures above 30°C on the viability of soilborne fungal pathogens (8,23,27,34). However, few studies have examined these effects on chlamydospores of Fusarium spp., which are more resistant to heat (2,8) and fumigants (10) than are conidia. Freeman and Katan (8) found that germination of F. oxysporum f. sp. niveum W.C. Snyder & H.N. Hansen conidia was delayed when spores were heated to 38 or 40°C for 30 to 90 min. Temperatures of 40°C for 120 min or 42°C for 60 min were required to obtain similar effects on chlamydospore germination (8). Although protocols for generating chlamydospores in media are available (1,9,19,22), a deterrent to studies of factors influencing chlamydospore survival may be the relative difficulty with which high numbers of chlamydospores can be produced, relative to the production of conidia. In addition, culture-produced chlamydospores may differ from chlamydospores produced naturally in plant Corresponding author: R. S. Bennett E-mail: [email protected] Current address: Wheat, Peanut and Other Field Crops Research Unit, USDA-ARS, Stillwater, OK 74075. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. The USDA is an equal opportunity provider and employer. Accepted for publication 6 May 2012. http://dx.doi.org/10.1094 / PDIS-09-11-0812-RE This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2012.