Fungi associated with die-back symptoms of apple and pear trees, a possible inoculum source of grapevine trunk disease pathogens
A survey was undertaken on apple and pear trees in the main pome fruit growing areas of the Western Cape of South Africa to determine the aetiology of trunk diseases with specific reference to pathogens known to occur on grapevine, which are frequently cultivated in close proximity to these orchards. Several fungal genera containing known trunk disease pathogens were found. Two Diplodia species, D. seriata and Diplodia sp., were isolated along with Neofusicoccum australe and N. vitifusiforme. Four Phaeoacremonium species, Phaeoacremonium aleophilum, Pm. iranianum, Pm. mortoniae and Pm. viticola, two Phomopsis species linked to clades identified in former studies as Phomopsis theicola and Phomopsis sp. 7, and Eutypa lata were found. In addition, Paraconiothyrium brasiliense, Pr. variabile and a Pyrenochaetalike species were also isolated. Diplodia seriata (56% of total isolates) and Pm. aleophilum (22%) were most frequently isolated. First reports from pear wood include the Phaeoacremonium spp. and Paraconiothyrium brasiliense, while new reports from apple include Pm. aleophilum, Ph. theicola, Phomopsis sp. 7, Pr. variabile and E. lata. A pathogenicity trial was undertaken to determine the role of these species on apple, pear and grapevine shoots. Neofusicoccum australe caused the longest lesions on grapevine shoots, while Pr. variabile, D. seriata, Pm. mortoniae and the Pyrenochaeta-like sp. caused lesions that were longer than non-inoculated and non-pathogen experimental controls. On pear shoots, Diplodia sp. and N. australe caused the longest lesions, followed by D. seriata and E. lata. On apple shoots, the longest lesions were caused by N. australe and Pm. iranianum. These results demonstrate that apple and pear trees in Western Cape orchards are hosts to many known trunk pathogens along with potential new trunk disease-causing fungi.