Pistachio (Pistacia vera) cultivation is a relatively new industry in South Africa with tremendous economic potential. However, disease problems could develop in time. The pathogens of pistachio found in other parts of the world plausibly will spread to South Africa and new pathogens never recorded on this host may develop here. In the summer of 1998, 2-year-old rootstocks of P. atlantica and P. integerrima displaying basal cankers and discolored phloem and xylem were observed in the Prieska district of the Northern Cape province. The objective of this study was to identify the causal agent(s). Cankered stems were split and small samples of discolored wood were aseptically placed on corn-meal agar containing streptomycin and incubated for 5 days. Fungi that grew into the agar were transferred to 1.5% water agar dishes containing pine needles to aid sporulation. Species isolated from stems included Fusarium spp. (44%), Fusicoccum aesculi (anamorph of Botryosphaeria dothidea) (23%), a Cytospora sp. (19%), a Chaetomium sp. (3%), and several nonsporulating mycelial fungi (11%). Inoculum of suspected pathogens (F. aesculi, the Cytospora sp., and two Fusarium spp.) were prepared by culturing fungi in petri dishes on potato dextrose agar overlaid with sterile cheesecloth strips (approximately 15 × 25 mm) until the strips were completely colonized. Greenhouse inoculations involved wounding P. atlantica stems by removing the bark (approximately 3 × 6 mm) approximately 10 cm above the soil, and wrapping the colonized cheesecloth followed by Parafilm around the stems at the wound site. Eight plants per isolate and eight wounded and nonwounded (untreated) control plants were used. Noncolonized cheesecloth was applied to wounded controls but not to nonwounded controls. Treatments were assigned randomly. Eight weeks after inoculation, the surrounding bark was removed from all treated shoots and the cambium was examined for discoloration. The length of each cambial lesion was measured and stem sections were surface disinfested and transferred to 1.5% water agar dishes, and the presence of the inoculated fungi was confirmed. Only F. aesculi was pathogenic to P. atlantica and produced cankers on all stems. The fungus was recovered from all tissues sampled 3 cm beyond the wound sites, 88% of those sampled from the wound sites, and none of the control treatments. Discoloration of the phloem and xylem was similar to that observed in the field. The mean canker length was 41 mm on trees with a mean stem diameter of 7 mm. The appearance of B. dothidea, in addition to B. obtusa (1), on pistachio justifies the need for establishing a disease management program for pistachio in South Africa.
Reference: (1) W. J. Swart and W.-M. Botes. Plant Dis. 79:1036, 1995.