Studies were established in 1992 and 1993 in a mature commercial pistachio orchard to determine the effectiveness of several fungicides for control of septoria leaf spot (Septoria pistaciurum). Fungicide treatments used in 1992 were Bravo 720F at 3.0 lbs./A (ai.) and 4.5 lbs./A a.i.; Kocide 101 50W at 8.0 lbs./A a.i. plus Benlate 50W at 1.0 lb./A a.i. Fungicide treatments in 1993 were Bravo 825 WDG at 3.0 and 4.5 lbs./A a.i. and Benlate 50W at 2.0 lbs./A a.i. Treatment replications consisted of two treated trees separated by nontreated trees within the row and nontreated tree rows dividing treated rows. At crop maturity, disease severity was determined by counting the number of leaf spots caused by septoria on ten leaves collected at random from each of the two trees of each replicated plot. All treatments significantly reduced disease severity compared to trees receiving no fungicide treatments. Experimental plots were too small to detect any apparent effect of fungicide treatments on yield. Leaves around nut clusters not receiving fungicide treatments were senescent at crop maturity, while leaves on treated trees showed no sign of senescence.
Robert E. Call and Michael E. Matheron
James D. McCreight, Michael D. Coffey, Thomas A. Turini and Michael E. Matheron
Races 1 and 2 of Podosphaera xanthii (syn. Sphaerotheca fuliginea) were defined in Imperial Valley, Calif. 1938 when P. xanthii overcame genetic resistance in `PMR 45'. Race 3 was first observed in the U.S. in 1976 in Texas; 15 additional races of P. xanthii have been reported in the literature since 1996. Races 1 and 2 have been common in Arizona and California based upon the effectiveness of the powdery mildew resistance genes in commercially available melon cultivars grown in these states. Field data from 11 commonly used melon P. xanthii race differentials in 2001 and 2002 indicated the presence of race 1 in the Imperial Valley and San Joaquin Valley of California, and Yuma, Arizona. In spring 2003, the powdery mildew race situation changed. The first evidence was the occurrence of a severe and widespread infection of powdery mildew in a commercial cantaloupe field. The 11 powdery mildew race differentials were susceptible to powdery mildew in a nearby replicated field test. PI 313970, a melon from India, was resistant to this apparent new race of powdery mildew.
James D. McCreight, Michael E. Matheron, Barry R. Tickes and Belinda Platts
Three races of Fusarium oxysporum f.sp. lactucae, cause of fusarium wilt of lettuce, are known in Japan, where the pathogen was first observed in 1955. Fusarium wilt first affected commercial U.S. lettuce production in 1990 in Huron, Calif., but did not become a serious problem in the U.S. until 2001 when it reappeared in Huron and appeared in the Yuma, Arizona lettuce production area. Reactions of three fusarium wilt differentials (`Patriot', susceptible to races 1, 2 and 3; `Costa Rica No. 4', resistant to race 1, and susceptible to races 2 and 3; and `Banchu Red Fire', susceptible to races 1 and 3, and resistant to race 2) in a naturally-infected commercial field test and artificially-inoculated greenhouse tests, indicated presence of race 1 in the Yuma lettuce production area. Reactions of these differentials to an isolate from Huron confirmed the presence of race 1 in that area. Consistent with previous results from the U.S. and Japan, `Salinas' and `Salinas 88' were resistant to the Yuma and Huron isolates of race 1, whereas `Vanguard' was highly susceptible. Limited F1 and F2 data indicate that resistance to race 1 in `Costa Rica No. 4' and `Salinas' is recessive. `Calmar' is the likely source of resistance in `Salinas' and `Salinas 88'.