Epidemiology and control of diseases caused by Alternaria species on pistachio
Alternaria spp. were isolated from various types of asymptomatic and symptomatic tissue of pistachio trees in South African orchards. Isolates were subdivided into four morphotypes based on colony morphology. Growth studies at different temperatures however, failed to differentiate the isolates into the same morphotypes. Fifty isolates from asymptomatic nuts, flower buds, leaves and twigs, as well as diseased nuts were submitted to a molecular assay using AFLP. These isolates were compared with five type species of Alternaria from the Centraalbureau voor Schimmelcultures. All Alternaria isolates from pistachio fell within a single A. alternata/A. arborescens group, and did not cluster according to the morphotypes. Due to the endophytic/quiescent nature of Alternaria spp. that cause disease on pistachio, the time of fungicide application is as critical as the formulation and concentration of the chemical used. Fungicide trails were conducted to determine the effect of repeated spraying throughout the year, making use of a system where all fungicides that proved to be effective against A. alternata isolates in vitro were sequentially used in a spray programme. Primary infection seemed to take place in the young developing flower buds. The use of fungicides early in the season is therefore critical if disease of the nuts is to be prevented. Further sequential sprays continued right up to the time of nuts being harvested. The two best performing fungicided treatments included all the fungicides that was found to be effective against Alternaria spp. in vitro, used sequencially from early in the season right up to harvesting. Doubling the dosage in the most successful application did not significantly reduce incidence of the fungus in nuts. Use of bud break oils as done in pistachio orchards in California and Australia was replaced by canopy wetting at Green Valley Nuts in order to increased the number of chilling hours needed for optimal fruit set. Isolations conducted from asymptomattic flower buds, asymptomatic midseason nuts or asymptomatic mature nuts revealed that none of the treatments increased the isolation frequency of quiescent Alternaria infection from pistachio tissue.