Effects of Water Stress on Botryosphaeria Blight of Pistachio Caused by Botryosphaeria dothidea

The effects of water potential (Ψ) on spore germination, germ tube elongation, mycelial growth of Botryosphaeria dothidea, and development of Botryosphaeria blight of pistachio caused by this fungus were studied in the laboratory and greenhouse. In laboratory tests, spore germination, germ tube elongation, and mycelial growth of B. dothidea increased as Ψ decreased from 0 to -2.0 MPa and declined as Ψ decreased below -2.0 MPa. Water stress (Ψ < -0.260 MPa) increased the severity of Botryosphaeria blight on young detached pistachio leaves. In greenhouse inoculation experiments, drought-stressed 2-year-old pistachio trees (leaf Ψ < -0.635 MPa) developed more severe Botryosphaeria blight disease than the nonstressed trees (Ψ > -0.485 MPa) in both 1999 and 2000 experiments. In a similar experiment using 1-year-old potted pistachio trees in 2000, withholding water treatments did not cause enough drought stress in pistachio leaves (Ψ > -0.584 MPa) and did not result in higher disease than the nonstressed trees (Ψ = -0.466 MPa). The correlation coefficient between Ψ and disease index for 1999 experiments was 0.954, and for 2000 experiments I and II was 0.981 and 0.939, respectively. These results indicate that drought stress can be a major predisposing factor of pistachio to infection by B. dothidea. Results from this study can be used to better understand the disease outbreaks in California pistachio orchards and to develop integrated management strategies by adjusting irrigation.