University of California at Davis, Kearney Agricultural Centre, 9240 S. Riverbend Ave., Parlier, California 93648, USA

Background and Objectives
Alternaria late blight caused by Alternaria alternata (Fr.) Keissler is a destructive disease of California pistachios. Although typical symptoms of Alternaria late blight appear later in the season (Aug-Sept), the disease is thought to develop from latent infections which occur on both the leaves and developing fruit during the early stages of the season (April-May). Typical late-season symptoms on leaves of both male and female trees are angular or circular dark brown to black necrotic lesions and later in the season, black sporulation can usually be observed at the center of the lesions. Multiple infections on leaf blades cause leaf blight and defoliation. Severe infections on the fruit can cause hull necrosis and lead to brown or black staining on the shell. Alternaria infection can also reduce fruit kernel quality as the pathogen is able to invade the kernel where it causes rot and the development of 'off' flavours. Control of Alternaria blight is problematic and continuing research is being carried out to optimize fungicidal and cultural control methods. The objective of our current study is to use microloggers to monitor environmental parameters in the field throughout the season and to assess and model disease development. We aim to produce a model which allows growers to maximize the efficacy of fungicidal spray treatments.

Materials and Methods
Microloggers (Campbell 21X) monitoring temperature, relative humidity (r.h.), leaf wetness and rainfall were placed in two orchards during April of 1997. One orchard (humid/warm) in east Kings County is flood irrigated with an underlying hardpan. This site has closely planted trees in narrow rows and a dense canopy which ensures high humidity. The second orchard (dry/hot) in southwest Kings County is microsprinkler irrigated, has free-draining sandy soil and younger trees planted in wide rows. The less-dense canopy ensures less humidity and warmer temperatures. Leaf and fruit samples were taken every 2 weeks and analysed for latent infections using an overnight freezing and incubation technique developed in our laboratory.

Results and Conclusions
Differences in the incidence and severity of latent infections of Alternaria late blight at the two site were correlated to differences in climatic parameters between the two sites. Latent infections at the dry, hot orchard did not increase significantly from low levels which occurred early in the season. Multiple regression analysis indicated that the significant increase in the incidence of latent infection on both leaves and fruit at the humid, warm orchard was associated with increased daily mean r.h. (R2=0.88, P<0.001), an increase in hours with r.h.>90% (R2=0.74, P<0.001) and hours of leaf wetness (R2=0.79, P<0.001) on and after 18 June, 1997. From this time on, which is generally the hottest part of the summer in California, dew forms most early mornings and late evenings, particularly in areas with high humidity and little breeze. The importance of relative humidity and leaf wetness (available free water) for the development of Alternaria late blight, as indicated by the results of this study agree with what is known of the epidemiology of Alternaria diseases in general [1] and the conclusions of previous studies with this pathogen on pistachio. Previous research indicated that reducing humidity in orchards during a 'critical period' at the beginning of August by omitting an irrigation leads to a reduction in the incidence and severity of this disease [2]. Similarly, converting a flood irrigated orchard to subsurface drip irrigation reduced the incidence of Alternaria late blight from 54% to 11% [3]. During the 1998 season, development of a predictive model will allow us to carry out a fungicide timing trial using 'spray-as-needed' applications as suggested by the model.

[1] Rotem J., 1994. The Genus Alternaria: Biology, Epidemiology and Pathogenicity. St Paul, MN: APS Press, pp. 326.
[2] Michailides TJ and Morgan DP, 1991. California Pistachio Industry Annual Report, Crop Year 1990-91, pp. 59-65.
[3] Michailides TJ, Morgan DP and Goldhamer DA, 1996. California Pistachio Industry Annual Report, Crop Year 1995-96, pp. 129-136.