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.

Gray mold, caused by the fungus Botrytis cinerea, usually has been considered a minor disease in field-grown lettuce. However, for the past two lettuce production seasons, the incidence of this disease has increased significantly in Southwestern Arizona lettuce fields. Most of the infected fields were planted to romaine lettuce; however, some iceberg lettuce plantings were involved as well. The occurrence of gray mold was most prevalent during February and March. The primary symptoms of the disease include plant wilting accompanied by a fuzzy gray growth at the plant base, which contains masses of airborne spores. Sclerotia may also be present on infected tissue. Infected plants that show these symptoms usually wilt and die. Botrytis cinerea can survive on crop debris, as a pathogen on numerous crops and weed hosts, and as sclerotia in soil. Airborne spores that land on senescent or damaged lettuce stems and leaves germinate and rapidly colonize this tissue. Once established, the pathogen grows into adjacent healthy stems and leaves. A field trial was established to 1) evaluate the effectiveness of some fungicides to reduce the incidence of Botrytis gray mold and 2) compare the susceptibility of some lettuce cultivars to this disease. Botrytis gray mold did not develop in plots of the crisphead cultivar 'Winterhaven'; however, the disease was present in all plots planted with cultivars of romaine lettuce. Among the tested romaine cultivars, Botrytis gray mold incidence was lowest in 'A 35585-1', 'Fresh Heart' and 'Rome 59' in untreated plots as well as in plots treated four times with the fungicides Rovral, Endura, or Switch. On the other hand, disease incidence was highest on the cultivar 'Green Towers' in untreated plots as well as those treated with one of the three tested fungicides. To compare the ability of tested fungicides to control Botrytis gray mold, the disease incidence values for each of the five romaine cultivars treated with each specific fungicide were pooled together and compared to disease levels recorded on untreated plants. Foliar applications of treatments were made Jan 24, Feb 1, Feb 16 and Feb 28, 2007. A 31% reduction in disease was recorded on romaine lettuce plants treated with Switch, whereas a 17% reduction in Botrytis gray mold was observed on plants treated with Endura. Rovral did not significantly reduce Botrytis gray mold in this trial. Additional field trials are planned to verify these initial findings.