Department of Plant Pathology

Powdery mildew is an important disease in commerciall members of the cucumber family. The specific pathogen that infects cucurbits in California is Podosphaera fusca (synonyms: P. xanthii, Sphaerotheca fulginea and S. fusca), (Janousek et al. 2009, McGrath and Thomas 1996, Pérez-García et al. 2009). Over-wintering chasmothecia produce ascospores that then develop into whitish colonies on leaves, leaf petioles, and stems (McGrath and Thomas 1996, Glawe 2008). Wind or insect vectors disperse asexually-produced conidia and thus spread the disease (Blancard et al. 1994). Favorable conditions for disease epidemics include temperatures between 20-27°C and lower-intensity light (McGrath and Thomas 1996). Disease outbreaks in the Central Valley of California tend to occur during late summer and autumn months, but coastal areas may be continuously threatened (Davey et al. 2008). Infections have the potential to reduce the yield and quality of fruit and can lead to early plant senescence (Blancard et al. 1994, McGrath and Thomas 1996).

Disease management in cucurbits usually involves foliar applications of synthetic fungicides and/or use of disease resistant cultivars (McGrath and Thomas 1996). Fungicides such as azoxystrobin, myclobutanil, quinoxyfen, trifloxystrobin, triflumizole, and micronized sulfur can be used to treat plants (Davis et al. 2008). Sulfur has the advantage of little or no risk of selecting for resistant mildew strains (Blancard et al. 1994). Previous work in our lab has shown that quinoxyfen, triflumizole, and penthiopyrad are highly effective at managing powdery mildew in disease susceptible varieties (Janousek et al. 2007, 2009).

We conducted two field trials at the UC Davis plant pathology experimental farm in Solano County, California to evaluate the effectiveness of ‘soft-chemistry’ and synthetic fungicides in managing powdery mildew on pumpkins and zucchini (Cucurbita pepo) using the susceptible cultivars Sorcerer and Elite, respectively. We applied fungicides every 7 to 14 days for a six week period beginning Sept 2 and continuing through Oct 13. Following four or seven applications, depending on treatment, we assessed disease incidence and powdery mildew colony density on the upper and lower surfaces of leaves in each treatment.

Bunch rot of grapes is caused by Botrytis cinerea, a fast-growing pathogen infecting numerous crops of commercial value. Bunch rot can potentially lead to a reduction in the yield and quality of table, raisin, and wine grapes, with high economic losses in some locations or years (Flaherty et al. 1992). Botrytis overwinters as sclerotia in mummified berries on the ground or on canes. The disease can first appear as shoot blight following frequent spring rains; flowers can become infected during bloom (Bulit and Dubos 1988). In infected fruits, disease symptoms are latent until late in the season. As sugar concentration increases in the berry, the fungus resumes growth and infects the entire fruit, often resulting in berry splitting and sporulation on the fruit surface (Flaherty et al. 1992). Free water is a requirement for the pathogen, and favorable conditions include humidities exceeding 90% and temperatures between 15-27°C (Flaherty et al. 1992, Bulit and Dubos 1988, Gubler et al. 2008). Along with leaf removal and other cultural controls, good spray coverage with a synthetic fungicide is currently the most effective form of disease management.

We examined the efficacy of 21 fungicide treatment programs for control of Botrytis bunch rot in Clone 4 Chardonnay grapes in Carneros, Napa County, California in 2010. Materials included synthetic, biological, and organic treatments. Four applications were made between June and September 2010.

Apple scab, caused by the fungal pathogen Venturia inaequalis, is a significant fruit and foliar disease worldwide (Jones and Sundin 2006). Apples grown in regions of California characterized by spring precipitation or damp microclimates are subject to infection. Initial pathogen colonization of green tissue occurs when water stimulates ascospore release from pseuodothecia located in overwintering leaf litter, followed by dispersal to leaves, flowers or fruit. Asexually-produced conidia from the primary sites of infection on the host can also colonize new tissue if spores are transported in the air or by water splash (Jones and Sundin 2006). In California, periodic applications of synthetic or organic fungicides from approximately March to June are required to control apple scab; the timing of fungicide applications is dependent on season to season patterns in precipitation (Gubler 2006). Based on research in other apple producing regions, additional control measures such as post-harvest fungicide applications at the time of leaf fall to reduce inoculum for the following growing season (Beresford et al. 2008), leaf litter removal (Gomez et al. 2007) or use of cultivar mixtures in an orchard (Didelot et al. 2007) may effectively reduce disease impacts.

We conducted a field experiment near Camino, El Dorado County, California (elevation 3200 ft) to test the effects of several registered and experimental fungicides on control of apple scab in mature Red Delicious Trees. Five applications were made from late March (green tip) to late May 2010 (post-bloom). We compared disease levels obtained on foliage in untreated trees with disease control exhibited by synthetic products in combination, with and without adjuvants, and in alternation with other products.

Powdery mildew is an economically-important pathogen of grapes worldwide. This report details the findings of our annual powdery mildew fungicide trials on grapevine cultivar Chardonnay (Vitis vinifera). The trials were conducted at Herzog Ranch, near Courtland, California in 2010. Treatments were placed in four adjacent trials in the vineyard. Spraying commenced in mid April, amidst significant rainfall events that likely promoted the release of powdery mildew (Erysiphe necator) ascospores from overwintering chasmothecia. Powdery mildew pressure began slowly with cool temperatures early on, but quickly built to very high disease pressure levels as temperatures warmed. Spraying was completed on July 23 and treatments were evaluated for disease incidence and severity.

Trial I consisted of soft chemistry products, including biologicals, sulfurs, nutrient applications, oils, and other materials. Spray frequencies varied from weekly applications to 21 day intervals. Many applications were based on the Gubler-Thomas Risk Index, with application intervals based on the index.

Temperatures were mild during much of the 2010 growing season, providing optimal conditions for the asexual reproduction and dispersal of powdery mildew. Overall disease pressure was higher than in similar trials conducted in 2007, 2008, and 2009. By late June, heavy to severe mildew coverage was evident on untreated clusters. Progression of the disease during spring was faster towards the eastern end of the research area. However, by the time of disease evaluation, disease severity in untreated plots in all four trials reached 95-100%.

Powdery mildew is a pervasive disease of cultivated grape. We conducted six field trials in a mature Chardonnay vineyard in Sacramento County to determine the efficacy of selected chemical and ‘soft chemistry’ fungicides (including new experimental materials) to control powdery mildew. Overall disease pressure at the site was low during much of the 2007 field season, but reached moderate levels by the time of veraison. Treatments exhibited a range of efficacy, with synthetic materials usually providing better control than copper-based products, biological fungicides, or other soft chemistry materials.