Spore release by Botryosphaeria dothidea in pistachio orchards and disease control by altering the trajectory angle of sprinklers

To design a disease-control management approach, release of pycnidiospores of Botryosphaeria dothidea from infected pistachio was studied in both laboratory and field. In laboratory studies, more cirrhi developed from pycnidia in pistachio rachises infected by B. dothidea that were dry in the field than in rachises washed several times during previous sprinkler irrigations. Pycnidia in older rachises produced fewer cirrhi than did younger rachises. Dry rachises released maximum numbers of pycnidiospores (up to 1,000 per milliliter of water) within 2-3 h of irrigation, and only three to four pycnidiospores were recovered after 10 h of sprinkling. In contrast, rachises with exuding pycnidia (cirrhi present on the surface of rachises) released 2.5 X 10(4), 1.3 X 10(4), and 63 pycnidiospores per milliliter of water 5, 30, and 450 min, respectively, after activating sprinklers. Released pycnidiospores from blighted shoots and petioles increased within 1-4 h of sprinkling and gradually decreased afterward, within 11-12 h of sprinkling. Released spores from blighted fruit increased gradually until 3 h after sprinkling, maintained high levels 4-9 h after sprinkling, and decreased 10-12 h after the first sprinkling. The numbers of released pycnidiospores decreased significantly with the second, third, fourth, and fifth sprinkling cycles, which were applied at weekly intervals. In field experiments, rachises collected from 1.2 to 1.5 m high in the tree canopy and "washed" in previous irrigations during the season released decreasing numbers of pycnidiospores with increases in sprinkling duration; "unwashed" rachises collected from 2.2 to 2.5 m high in the tree canopy released increasingly high levels of pycnidiospores until 5 h after sprinkler irrigation. In inoculation experiments, higher pycnidiospore concentrations resulted in greater incidence and severity of disease than resulted from lower concentrations, in both male and female pistachio. Altering the irrigation-sprinkler trajectory angle from 23 degrees to 12 degrees significantly reduced the disease in three commercial orchards in California.