Phytophthora

California Avocado Society 1994 Yearbook 78:121-129

Comparison of Resistance to Phytophthora citricola in Nineteen Avocado Rootstocks under Greenhouse Conditions
Zeinab A. El-Hamalawi, John A. Menge, and Fred B. Guillemet Department of Plant Pathology, University of California, Riverside, CA 92521 Abstract Nineteen vegetatively-propagated 8-mon-old avocado (Persea americana) rootstockselections (Aguacate mico, Anaheim, Borchard, D9, Duke 6, Dusa, G22, G582, G1033, Golden, Hibbard, Thomas, UC2001, UC2002, UC2003, UC2004, UC2011, Evstro, and Velvick) were evaluated for resistance to Phytophthora citricola in the greenhouse. Two weeks after stem inoculation with P. citricola, the canker size was determined. The relative resistance of the rootstock selections to infection with P.citricola was calculated relative to Hibbard rootstock which demonstrated the highest mean canker size of all rootstocks tested. The rootstocks Thomas and UC2004 demonstrated the second lowest relative resistance among the rootstocks. A group of twelve rootstocks including Borchard, UC2002, UC2003, Dusa, D9, Duke 6, UC2011, G22, Evstro, Aguacate mico, UC2001, and G1033 showed the highest relativeresistance (74.5-84.5%) to P. citricola. The rootstocks G592, Anaheim, Golden, and Velvick were intermediate in their level of relative resistance compared with other rootstocks. Introduction Avocado (Persea americana Miller) trunk canker disease, which is commonly known as citricola canker, is caused by Phytophthora citricola Sawada. P. citricola is causing increasing damage to avocado groves throughoutCalifornia in recent years (1). P. citricola affects the crown, lower trunk, and sometimes the main structural roots (1, 10). The typical symptoms of the disease include bark cracking and exudation of a white, sugary material, usually at the base of the trunk. In advanced stages, defoliation and twig dieback occur. If the canker encircles the trunk the tree will die. Phytophthora citricolareadily produces thick-walled oospores in culture. It also produces sporangia which liberate motile zoospores. It is very likely that because of its ability to produce zoospores, P. citricola can establish itself and spread throughout avocado groves or nurseries; and because of its abundant oospores, it is very difficult to control. Oospores are the most resistant spore structure produced byPhytophthora spp. and may survive in soil in the absence of a host for more than one year. The fungus has been also detected in feeder roots and main roots of avocado (2). Currently, the use of clonally-propagated, moderately resistant rootstocks represents an important component of the integrated approach to control Phytophthora cinnamomi in avocados (1, 9). This approach has been used to develop avocadorootstocks with field

resistance to P. cinnamomi by screening more than 3,000 selections of different Persea spp. for root rot resistance (11, 12). Because of the potential threat of P. citricola to the avocado industry in California, the purpose of the work reported here was to screen a number of the common, commercially available avocado rootstock cultivars for high levels of resistance toinfection by Phytophthora citricola. Materials and Methods Rootstock propagation. The seedlings of P. americana cv. Lula were grown from seed planted in flats containing sand or vermiculite in the greenhouse at 24 ± 2°C. While still small, seedlings were transplanted individually into paper pots (10-cm diam., 10 cm depth) filled with UC-soil mix. Nineteen rootstock selections (Table 1) werepropagated vegetatively. Budwood from each clonal selection was tip-grafted to seedlings of P. americana cv. Lula as nurse plant. Shoot etiolation and subsequent root production allowed the establishment of clonal plants using a delicate procedure developed by Frolich and Platt (1971). Large clonal rootstocks were transplanted into plastic pots (15 cm diam., 17 cm depth) containing peat/vermiculite (1:1)...