Determining the chilling requirement for ‘Sirora’ pistachio (Pistacia vera L.) production in Australia and monitoring winter chill accumulation is desirable to allow growers to take timely mitigating action in years of insufficient chill. Based on greenhouse work, historical data analysis, and field validation, the Chilling Hour, Utah, and Dynamic Models were compared. The Dynamic Model produced the best determination for fulfillment of chilling requirement with 59 chill portions. The required number of growing degree-hours above 4.4 °C from chill fulfillment to 50% bloom was 9633. Relations among the level of fulfillment of chilling requirement, bloom delay, and yield are discussed.

Symptoms associated with pistachio dieback in Australia include decline (little or no current season growth), xylem staining in shoots two or more years old, trunk l and limb lesions (often covered by black, superficial fungal growth), excessive exudation of resin, dieback and death of the tree. Bacteria belonging to the genus Xanthomonas have been suggested as the causal agent. To confirm the constant association between these bacteria and the disease syndrome, the absence of other pathogens and the identity of the pathogen, we performed a series of isolations and pathogenicity tests. The only microorganism consistently isolated from diseased tissue was a bacterium that produced yellow, mucoid colonies and displayed morphological and cultural characteristics typical of the genus Xanthomonas. Database comparisons of the fatty acid and whole-cell protein profiles of five representative pistachio isolates indicated that they all belonged to X. translucens, but it was not possible to allocate the isolates to pathovar. Pathogenicity tests on cereals and grasses supported this identification. However, KochÕs postulates have been only partially fulfilled because not all symptoms associated with pistachio dieback were reproduced on inoculated twoyear-old pistachio trees. While discolouration was observed, dieback, excessive resinous exudate and trunk and limb lesions were not produced; expression of these symptoms may be delayed, and long-term monitoring of a small number of inoculated trees is in progress.

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Xanthomonas translucens has been identified as the causal agent of pistachio dieback in Australia. Symptoms include decline, xylem staining, trunk and limb lesions, and excessive exudation of resin. Bacteria were previously isolated from stained wood in 2 year old twigs but little was known about their presence in other parts of the tree. The pattern of staining and location of X. translucens were studied following felling and dissection of asymptomatic and diseased trees. Chestnut-coloured smears and specks occurred in the 3 sapwood of diseased trees and were continuous from the trunk to 1-2 year old twigs. Xanthomonas translucens was isolated mainly from young sapwood (stained and unstained) of the main trunk, primary and younger branches and current season growth, less frequently from leaves and bunches, rarely from old, stained heartwood and not from roots and associated soil samples. Bacteria and pathogenic fungi were not found in the inner bark and cortex associated with lesions whereas the stained sapwood underlying the lesions yielded X. translucens. Scanning electron microscopy revealed bacteria in the main vessels of the xylem of stained tissue and tyloses in the proximity of colonised tissue. Information on the pattern of staining and location of the bacteria will facilitate pathogen detection, thereby improving the accuracy of disease diagnosis.

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