Disease Note.

Occurrence of Sclerotinia and Botrytis Shoot Blights on Pistachio in California. T. J. Michailides, University of California, Davis/Kearney Agricultural Center, Parlier 93648. D. P. Morgan, University of California, Davis/Kearney Agricultural Center, Parlier 93648. Plant Dis. 78:641. Accepted for publication 10 January 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0641D.

Sclerotinia shoot blight, caused by Sclerotinia sclerotiorum (Lib.) de Bary, was first recorded in 1986 in two commercial pistachio (Pistacia vera L.) orchards in the San Joaquin Valley and in three orchards in the Sacramento Valley. Isolations from blighted current-growth shoots, collected from nine surveyed orchards, consistently revealed either S. sclerotiorum (in five orchards) or Botiytis cinerea Pers.:Fr. (causing Botrytis shoot blight [1]) lo be present in all orchards. Prewounded current-growth shoots of the pistachio cultivar Kerman were inoculated with mycelial plugs from PDA cultures of two isolates of S. sclerotiorum collected from different locations. All shoots inoculated in April and early May showed leaf wilting within 2 days, and they were killed 7-10 days after inoculation. Shoots inoculated from the end of May to the end of July had 50-70% blight after 1-2 wk, and none were killed until 2 mo after inoculation. Symptoms on blighted twigs artificially inoculated with S. sclerotiorum resembled those on naturally infected twigs due to either S. sclerotiorum or B. cinerea, but reisolation from these twigs yielded only S. sclerotiorum. Twig infections caused by B. cinerea could sometimes be distinguished from those of S. sclerotiorum by the buff-colored sporulation of B. cinerea, especially in cool wet weather. Frequently, however, no distinguishing signs were present lo separate the two causal organisms. For correct diagnosis, therefore, isolation from blighted twigs that lack sporulation of ll. cinerea is required. In 1992, samples of blighted shoots were collected from a commercial orchard, and S. sclerotiorum was isolated from 80% of the samples. Although not as common as Botrytis blight, Sclerotinia shoot blight can become a major disease of pistachio in California under certain conditions, and methods for control should be considered.