Identification and Pathogenicity of Fungal Pathogens Associated with Stem-End Rot of Avocado in California

Twizeyimana, M., Forster, H., McDonald, V., Wang, D. H., Adaskaveg, J. E., and Eskalen, A. 2013. Identification and pathogenicity of fungal pathogens associated with stem-end rot of avocado in California. Plant Dis. 97:1580-1584. Stem-end rot of harvested avocado fruit commonly occurs wherever the crop is cultivated. Multiple fungal species have been described as causal agents. To determine the causal pathogens of stem-end rot in California, fungal isolations were conducted from symptomatic fruit, and fungi were identified by morphological and molecular techniques. In 2010 and 2011, a total of 177 isolates were recovered from 290 avocado fruit collected from seven orchards in one of the major avocado growing areas in Southern California. The majority of isolates was identified as Neofusicoccum luteum (65%), with the remainder either as Colletotrichum gloeosporioides (33%) or Phomopsis sp. (2%). In a pathogenicity test, N. luteum caused significantly (P 0.05) differences in stem-end rot severity were observed between inoculations with N. luteum isolated from fruit stemend rot and N. luteum or N. parvum isolated from branch cankers. This confirms that stem-end rot of avocado can be initiated by fungi causing branch cankers. Although low humidity and rainfall during much of the growing and harvest seasons in California are considered unfavorable conditions for the development of avocado stem-end rot, the identification of the causal pathogens is of value when decays have to be managed during outbreaks, and it stresses the importance of managing branch cankers. Stem-end rot of avocado (Persea americana Mill.) has been reported from all major production areas worldwide (12,17). Crop losses may be severe when harvest occurs during rainy periods (9). The first visible symptom of stem-end rot is a shriveling around the stem base that is followed by decay, fruit discoloration, and softening. The decay ultimately consumes the entire fruit. Internally, the decayed flesh is brown and the vascular bundles in advance of the decay may darken. Despite the long period of exposure to potential pathogens, fruit rarely display symptoms before harvest. Thus, in commercial avocado production, stem-end rot mostly occurs during transit, in the packinghouse, and during or after marketing. Various fungi cause stem-end rot of avocado. They include species in the Botryosphaeriaceae such as Botryosphaeria lutea A.J.L. Phillips (anamorph Neofusicoccum luteum (Pennycook & Samuels) Crous, Slippers & A.J.L. Phillips), Botryosphaeria dothidea (Moug. ex Fr.) Ces. & De Not. (anamorph Fusicoccum aesculi Corda), B. parva Pennycook & Samuels (anamorph Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers & A.J.L. Phillips), B. ribis Grossenb. & Duggar (anamorph Neofusicoccum ribis (Slippers, Crous & M.J. Wingf.) Crous, Slippers & A.J.L. Phillips), and B. rhodina (Berk. & M.A. Curtis) Arx (anamorph Lasiodiplodia theobromae (Pat.) Griffon & Maubl.). Other species reported include Glomerella cingulata (Stoneman) Spauld. & H. Schrenk (anamorph Colletotrichum gloeosporioides (Penz.) Penz. & Sacc.), Glomerella acutata Guerber & J.C. Correll (anamorph C. acutatum J.H. Simmonds), Nectria pseudotrichia (Schwein.) Berk. & M.A. Curtis, and species of Albonectria, Alternaria, Gibberella, Pestalotiopsis, Phomopsis, and Rhizopus (12,13,17,20–22,26). Colletotrichum spp. can also cause fruit anthracnose (24). Botryosphaeriaceae spp., which are generally recovered in greater numbers from stem-end rot of avocado fruit than other fungi and are often associated with severe rots (26), vary by