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

[摘要] English: Drought, freezing and nitrogen stress are environmental factors known topredispose woody plants to infection by numerous fungal species. The main and ultimateobjective of the present study was to identify possible stress factors that predisposecommercially pistachio (Pistacia spp.) trees to infection and colonization byBotryosphaeria obtusa and B. dothidea in order to develop a sustainable disease controlprogramme for South African pistachio producers.It was found that these stress conditions significantly affected the susceptibility ofthree genotypes of potted pistachio plants to infection and colonization by B. obtusa andB. dothidea.In the glasshouse, drought stress trials with potted Pistacia terebinthus plantsartificially inoculated with B. dothidea showed significantly longer lesions than controltreatments while P. integerrima displayed the greatest resistance to the pathogen.Although B. dothidea caused more severe cambial lesions in glasshouse trials, it showeda slower growth rate than B. obtusa under in vitro water stress conditions.In freezing stress trials performed in a growth chamber, there was a highlysignificant difference between the stressed and non-stressed plants of P. integerrima andClone II (P. integerrima x P. atlantica). P. integerrima was the most susceptiblerootstock and P. terebinthus the least susceptible. B. dothidea caused the most severecambial lesions in all treatments.In nitrogen stress trials, most treatments showed a significant decrease in thelength of cambial lesions at a higher level of applied nitrogen (28%). B dothidea wasgenerally more pathogenic than B. obtusa for both levels of N applied. The mostsignificant differences between nitrogen levels were recorded for P. terebinthus.From the above results it is clear that drought and freezing stress have asignificant 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 periodlasting two to three weeks. By postponing this period not only can drought stress beprevented but also freezing stress. Current nitrogen levels applied in the pistachioorchards are much lower than those applied in other parts of the world. Applying higherlevels 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 fromdebri, 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 latentphase.Trials were conducted in vitro with different fungicides to determine otherpossible fungicides that could be employed for the control of the two Botryosphaeriaspecies. Eria®, a mixture of carbendazim and difenoconazole displayed the highestinhibition. Benomylor the Eria® applied during bloom, and tebuconazole in combinationwith difenoconazole on a monthly basis throughout the season, should thus successfullycontrol both Botryosphaeria diseases in South African orchards.

[发布机构] University of the Free State