First Report of Rhizoctonia solani (AG-4) on Pistachio Rootstock Seedlings. B. A. Holtz, University of California Cooperative Extension, Madera 93637. T. J Michailides and L. Feguson, University of California, Davis/Kearney Agricultural Center, Parlier 93648; and J. G. Hancock and A. R. Weinhold, University of California, Berkeley 94720. Plant Dis. 80:1303. Accepted for publication 6 September 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1303A.
In March 1995, pistachio rootstock seedlings, Pistacia atlantica, P. integerrima (Pioneer Gold I [PG I]), and a hybrid called UCB I (P. atlantica female crossed with P. integerrima male), were reported to be stunted, dying, or killed in a nursery in Madera County. Over 10,000 seedlings were lost. The leaves of infected seedlings turned brown, withered, and clung to the shoots. Brown conical lesions developed on the roots and roots became discolored. Isolations from roots of dying seedlings resulted in the recovery of Rhizoctonia solani Kuhn. The isolates were assigned to anastomosis group 4 (AG-4) after hyphal fusion was observed in pairings with known strains. The pathogenicity of three isolates was tested in the greenhouse (15 to 25°C) against 3- to 4-week-old healthy P. atlantica, P. integerrima (PG I), and UCB I seedlings by mixing 50 ml of 2-week-old cornmeal/sand inoculum (3 g of commeal, 100 g of sand, and 20 ml of distilled water) into 9-cm-diameter plastic pots containing a steamed soil/peat/perlite mixture (2:2:1, vol/vol) and one rootslock seedling per pot. Six pots were used per isolate, two pots per rootstock. Six pots containing noninfested soil and rootstocks were evaluated for comparison. Two weeks later symptoms were apparent and roots were evaluated for disease severity. Brown, discolored lesions were observed on the roots of all three rootstocks, and Koch's postulates were completed by reisolating all three isolates from root lesions. The experiment was repeated with similar results. The disease was controlled in the nursery with the addition of pentachloronitrobenzene (PCNB) into the planting medium and by maintaining clean greenhouse benches.