Further evaluation of Soil Solarization as a Management Tool for Fusarium Wilt of Lettuce: 2006 Field Trial

Abstract

Fusarium wilt of lettuce was first recognized in Arizona in 2001. Since this initial discovery, the pathogen, Fusarium oxysporum f.sp. lactucae (Fol), has been recovered from infected lettuce plants from over 40 different fields. This fungus is a soil-borne pathogen that can remain viable in soil for many years. Soil solarization has shown promise in managing Fusarium wilt in other cropping systems as well as in lettuce field trials conducted during 2004 and 2005. In an effort to maximize the solarization effect on subsequent disease development, the following factors were evaluated in a 2006 field trial: 1) solarization of unshaped versus preshaped beds, 2) the effect of soil moisture on solarization efficiency of preshaped beds, and 3) effect of lettuce type on Fusarium wilt incidence after solarization. The entire field was flood irrigated on Jun 21. Plots were solarized during Jul and/or Aug by covering beds with 1-mil thick clear plastic. During the solarization treatment from Jul 3 to Sep 10, the mean soil temperature in preshaped solarized beds at a depth of 2 and 9 inches was 116 and 95°F, respectively, and 102 and 97°F, respectively, in beds not covered with plastic. When solarization was initiated 15 days after soil irrigation, a 20% reduction in Fusarium wilt was recorded for a crisphead lettuce cultivar grown on solarized unshaped beds compared to a 56% reduction in disease when the same crisphead cultivar was grown on preshaped solarized beds. There was no significant difference between a one and two month solarization period in the subsequent number of diseased lettuce plants. Solarization of preshaped beds 15 days after irrigation for one month reduced Fusarium wilt on crisphead lettuce by 56%, whereas the same solarization period initiated seven days after irrigation resulted in a 96% reduction of disease. The same one-month solarization period started one week after soil irrigation reduced the incidence of Fusarium wilt on green leaf (Two Star) and romaine (Green Towers) by 97 and 88%, respectively, compared to plants grown on unsolarized beds. The data show that summer solarization of moist soil can 1) destroy propagules of Fusarium oxysporum f. sp. lactucae in infested fields and 2) be a useful cultural management tool to significantly reduce the incidence of Fusarium wilt in a subsequent crop of lettuce.