Drought, freezing, and nitrogen stress as factors predisposing pistachio trees to infection by Botryosphaeria obtusa and B. dothidea

English: Drought, freezing and nitrogen stress are environmental factors known to predispose woody plants to infection by numerous fungal species. The main and ultimate objective of the present study was to identify possible stress factors that predispose commercially pistachio (Pistacia spp.) trees to infection and colonization by Botryosphaeria obtusa and B. dothidea in order to develop a sustainable disease control programme for South African pistachio producers. It was found that these stress conditions significantly affected the susceptibility of three genotypes of potted pistachio plants to infection and colonization by B. obtusa and B. dothidea. In the glasshouse, drought stress trials with potted Pistacia terebinthus plants artificially inoculated with B. dothidea showed significantly longer lesions than control treatments while P. integerrima displayed the greatest resistance to the pathogen. Although B. dothidea caused more severe cambial lesions in glasshouse trials, it showed a slower growth rate than B. obtusa under in vitro water stress conditions. In freezing stress trials performed in a growth chamber, there was a highly significant difference between the stressed and non-stressed plants of P. integerrima and Clone II (P. integerrima x P. atlantica). P. integerrima was the most susceptible rootstock and P. terebinthus the least susceptible. B. dothidea caused the most severe cambial lesions in all treatments. In nitrogen stress trials, most treatments showed a significant decrease in the length of cambial lesions at a higher level of applied nitrogen (28%). B dothidea was generally more pathogenic than B. obtusa for both levels of N applied. The most significant differences between nitrogen levels were recorded for P. terebinthus. From the above results it is clear that drought and freezing stress have a significant effect on the predisposition of pistachio plants to B. obtusa and B. dothidea. Drought stress occurs in pistachio orchards due to a very short hardening off period lasting two to three weeks. By postponing this period not only can drought stress be prevented but also freezing stress. Current nitrogen levels applied in the pistachio orchards are much lower than those applied in other parts of the world. Applying higher levels of nitrogen could thus possibly prevent the occurrence of Botryosphaeria diseases. The development of a selective medium for the pure isolation of B. obtusa and B. dothidea from diseased tissue and other sources enabled the isolation of B. dothidea from debri, bark, diseased and asymptomatic tissue without hindrance by extraneous fungi. B. obtusa, however, was only isolated from active cankers on the stems of pistachio trees. Isolation of B. dothidea from asymptomatic plant tissue suggests an endophytic or latent phase. Trials were conducted in vitro with different fungicides to determine other possible fungicides that could be employed for the control of the two Botryosphaeria species. Eria®, a mixture of carbendazim and difenoconazole displayed the highest inhibition. Benomylor the Eria® applied during bloom, and tebuconazole in combination with difenoconazole on a monthly basis throughout the season, should thus successfully control both Botryosphaeria diseases in South African orchards.