PRO/PL> Fusarium wilt, lettuce - USA (AZ): first report

FUSARIUM WILT, LETTUCE - USA (ARIZONA): FIRST REPORT
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Date: 22 Sep 2003
From: ProMED-mail<[EMAIL PROTECTED]>
Source: American Phytopathological Society, PLANT DISEASE NOTES [edited]


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.

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[In USA, Fusarium wilt [Fw] of lettuce was first found in lettuce 
fields in Fresno County near Huron, CA in 1990 and subsequently in 
Yuma County, AZ in 2001-2002 in Watsonville, CA. Fw spores can remain 
in soil and infected crop material for many years. 11 new infected 
lettuce fields were found in the 2002-2003 season in Arizona. 
Scientists at the Yuma Agricultural Center have noted varietal 
differences in susceptibility to Fw. Crop losses were 95, 75, 60, 50, 
and 18 percent in head lettuce, butterhead, greenleaf, redleaf, and 
romaine, respectively. Gross income for head lettuce producers was 
$329 million dollars (US) in 2001. Recommendations for disease 
management include preventing spread of contaminated soil on field 
equipment and planting in fields known  to be free of the pathogen. A 
search for genetic resistance to Fw has begun. - Mod.DH]

[see also:
2002
----
Fusarium wilt, lettuce - Europe 20020821.5103
Fusarium wilt, cucumber - Spain (Almerma) 20021109.5759]
.......................................dh/pg/lm
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