First Report of Neofusicoccum australe, N. luteum, and N. parvum Associated With Avocado Branch Canker in California

Abstract

In 1953, branch cankers on California avocado (Persea americana Mill.) trees were attributed to a Botryosphaeria anamorph, Dothiorella gregaria (teleomorph B. ribis) (2), and the disease was known as Dothiorella canker. Since this time, it has been suggested that this fungus should probably be classified as Fusicoccum aesculi Corda (teleomorph B. dothidea) (3). To our knowledge, B. dothidea is the only reported Botryosphaeriaceae species causing Dothiorella canker on avocado in California. Between the summer of 2008 and the winter of 2009, five trees from each of eight avocado orchards in five counties (San Diego, Riverside, Ventura, Santa Barbara, and San Luis Obispo) were surveyed for Dothiorella canker symptoms to verify the associated Botryosphaeriaceae species. Typical Dothiorella canker symptoms observed included darkened and friable bark with a dried, white, powdery exudate. Underneath the bark, cankers were variable in shape and some penetrated into the heartwood. Small sections of tissue (0.5 cm2) were excised from two to four separate cankers per tree and placed onto potato dextrose agar amended with tetracycline (0.01%) (PDA-tet). The most frequently isolated fungi, based on general growth pattern, speed, and colony color, were in the Botryosphaeriaceae with the following percent recovery by county: Riverside–40 and 100% (site 1 and 2, respectively); San Diego–60% (site 3); Ventura–42 and 53% (site 4 and 5, respectively); Santa Barbara–33% (site 6); and San Luis Obispo–32 and 60% (site 7 and 8, respectively). Pycnidia of Botryosphaeriaceae species were also observed on old diseased avocado tree branches. Sequenced rDNA fragments (ITS1, 5.8S rDNA, ITS2, amplified with ITS4 and ITS5 primers) were compared with sequences deposited in GenBank. Four different Botryosphaeriaceae species were identified and included Neofusicoccum australe, B. dothidea, N. luteum, and N. parvum, with species nomenclature based on the work of Crous et al. (1). Pathogenicity tests were conducted in the greenhouse on 1-year-old avocado seedlings, cv. Hass, with one randomly chosen isolate from each of the Botryosphaeriaceae species noted above. Four replicate seedlings were stem-wound inoculated with a mycelial plug and covered with Parafilm. Sterile PDA plugs were applied to four seedlings as a control. Over a period of 3 to 6 months, seedlings were assessed for disease symptoms that included browning of leaf edges and shoot dieback. Mean vascular lesion lengths on stems were 64, 66, 64, and 18 mm for B. dothidea, N. parvum, N. luteum, and N. australe, respectively. Each fungal isolate was consistently reisolated from inoculated seedlings, thus fulfilling Koch's postulates. To our knowledge, this is the first report of N. australe, N. luteum, and N. parvum recovered from branch cankers on avocado in California. These results are significant because Botryosphaeriaceae canker pathogens are known to enter the host plant through fresh wounds (pruning, frost, and mechanical). With high-density planting becoming more common, which requires intensive pruning, the transmission rate of these pathogens could increase in California avocado groves. References: (1) P. W. Crous et al. Stud. Mycol. 55:235, 2006. (2) F. F. Halma and G. A. Zentmyer. Calif. Avocado Soc. Yearb. 38:156, 1953. (3) W. F. T. Hartill and K. R. Everett. N. Z. J. Crop Hortic. Sci. 30:249, 2002.

Characterisation and pathogenicity of fungal species associated with branch cankers and stem-end rot of avocado in Italy

“…This fungus was recovered from all orchards sampled and was identified by comparing morphology and through phylogenetic analysis (Phillips et al 2013). Neofusicoccum parvum is reported to be a major stem canker and stem-end rot pathogen of avocado in California, Australia, New Zealand and South Africa (Darvas et al 1987;Hartill and Everett 2002;McDonald et al 2009;Twizeyimana et al 2013 (Ismail et al 2013, Phillips et al 2013. Indeed, this is the first report of N. parvum causing disease on avocado (branch canker and stem-end rot) in Italy.…”

Fungal trunk pathogens associated with wood decay of pistachio trees in Iran

“…Recently, B. dothidea has been isolated and reported from pistachio trees showing panicle blight in Iran (Mohammadi et al, 2015). Neofusicoccum parvum has been isolated from pome and stone fruit trees in South Africa , from avocado in California (McDonald et al, 2009), from almond (Inderbitzin et al, 2010), English walnut (Chen et al, 2014a), and grapevine in California (Úrbez-Torres & Gubler, 2009). In Iran, both species have been isolated from cypress and grapevine (Arabnezhad & Mohammadi, 2012;Mohammadi et al, 2013b).…”

A nested multiplex PCR for species-specific identification and detection of Botryosphaeriaceae species on mango

“…Botryosphaeria is a species-rich genus, and has been associated with various plant disease symptoms such as pistachio shoot blight , eucalyptus canker and shoot blight (Yu et al 2009), mango die-back (Javier-Alva et al 2009), avocado branch canker (McDonald et al 2009), avocado dieback (Zea-Bonilla et al 2007), grapevine cankers (Urbez-Torres 2006), mango stem-end rot and fruit rot (Jacobs 2002;Ni et al 2010;Slippers et al 2005), and avocado fruit rot (Ni et al 2009). These fungal species are easily distinguished from most other fungi by their grey to black, aerial mycelium, and grey to indigogrey or -black pigment that is visible from the reverse side of Petri dishes (Slippers and Wingfield 2007).…”