While pistachio growers this year received a welcomed Section 18 emergency registration for Rovral for alternaria late blight, no fully registered products offer 100 percent control. Rovral, applied in two July applications at the full dosage of 2 pounds per acre is effective, but there is no guarantee that control measure will continue to be available. Alternaria blight is considered the second worst problem to pistachio growers next to verticillium wilt, and is becoming more and more prevalent every year, according to Plant Pathologist Themis Michailides at UC's Kearney Ag Center in Parlier. Michailides said Kerman pistachios are particularly vulnerable to alternaria, although all varieties are susceptible. The blight shows different symptoms for different varieties, although the initial symptom to commonly occur is a premature defoliation of leaves.
Lesions begin to develop on leaves shortly thereafter.
Blackened hulls usually indicate the shell and nut meat will be stained or moldy. The critical period for blight development is from Aug. 11 through Sept. 30 when higher humidity and dew at night create colonization of fungus spores. Fruit is also more susceptible to the disease in August when it is mature. Nichols Farms in Kings County, Calif., decided to look at subsurface drip irrigation to reduce the disease in one particular orchard that showed severe alternaria late blight. The 80-acre orchard, Sierra View Farms in Hanford had been on flood irrigation, but heavy soils with poor infiltration left standing water that not only made cultural practices difficult, but created a humid environment ripe for the development of alternaria fungus.