First report of Fusarium Wilt of lettuce caused by Fusarium oxysporum f. sp. lactucae in Arizona

M. E. Matheron, University of Arizona, Yuma Agricultural Center, Yuma 85364; and S. T. Koike, University of California Cooperative Extension, Salinas 93901. Plant Dis. 87:1265, 2003; published on-line as D-2003-0806-01N, 2003. Accepted for publication 23 Jul 2003.

A new wilt and root rot disease was observed in 6 and 11 commercial fields of lettuce (_Lactuca sativa_) in western Arizona during the fall of 2001 and 2002, respectively. Distance between infested sites ranged from approximately 0.5 to 39 km. 5 head lettuce cultivars as well as a red leaf lettuce cultivar were affected.

Disease symptoms included yellowing and wilting of leaves, as well as stunting and plant death. The cortex of the crown and upper root of infected plants usually was decayed and reddish brown. Disease symptoms first appeared at the time of plant thinning and continued to develop up to plant maturity. _Fusarium oxysporum_ was consistently isolated from symptomatic plant roots.

Seeds of cv. Lighthouse were planted in nonsterile vermiculite within 3.0-cm-square x 7.0-cm-deep cells in a transplant tray and thinned to a single plant per cell. When the first true leaves were emerging, 10 individual seedlings were inoculated with a single-spore isolate of _F. oxysporum_ recovered from diseased lettuce root cortex tissue.

Inoculum was prepared by growing the fungus on potato dextrose agar in 100-mm-diameter x 15-mm-deep plastic petri dishes at 28 deg C with a 12-h photoperiod under fluorescent light. Once the fungus completely covered the agar surface, 50 ml of sterile distilled water was added to the dish, and the mycelia and conidia on the surface were scraped off the agar and suspended in the water. This fungal suspension was decanted, and a 2-ml aliquot containing 180 000 CFU was pipetted into the vermiculite near the stem of each lettuce seedling.

10 plants grown in noninfested vermiculite served as uninoculated controls. After inoculation, plants were maintained in a growth chamber at 28 deg C with a 12-h photoperiod under fluorescent light for 3 weeks. Symptoms of yellowing, wilt, vascular decay, and often plant death developed during the incubation period on all inoculated plants but not on control plants.

_Fusarium oxysporum_ was consistently reisolated from inoculated plants but not from uninoculated plants. The experiment was repeated and yielded the same results.

A wilt and root rot disease of lettuce attributed to _F. oxysporum_ f. sp. _lactucae_ was first reported in Japan in 1967 (3) and subsequently in the United States (San Joaquin Valley of California) in 1993 (2), and Italy in 2002 (1). The researchers of the U.S. report did not cite the earlier work from Japan and described the pathogen as _F. oxysporum f. sp. lactucum_. The Arizona isolate used to demonstrate pathogenicity was of the same vegetative compatibility group as an isolate of the pathogen from lettuce in California reported in 1993.

Several companies grow and harvest lettuce in Arizona and California. At the end of production and harvest in the fall, tractors, implements, and harvesting equipment are transported from the San Joaquin Valley in California to western Arizona. The similarity between the isolate of _F. oxysporum_ f. sp. _lactucae_ from western Arizona and the San Joaquin Valley of California suggest a possible introduction of the pathogen into Arizona from California, perhaps on soil adhering to farm equipment.

To our knowledge, this is the first report of _F. oxysporum_ f. sp. _lactucae_ infecting lettuce in Arizona.

References:
(1) A. Garibaldi et al. Plant Dis. 86:1052, 2002.
(2) J. C. Hubbard and J. S. Gerik. Plant Dis. 77:750, 1993.
(3) T. Matuo and S. Motohashi. Trans. Mycol. Soc. Jpn. 8:13, 1967.