Integrated approach to controlling Diaporthe canker of deciduous fruit in South Africa

Diaporthe canker is a newly recognized fungal disease affecting the longevity and productivity of deciduous fruit orchards in the Cape Province of South Africa. A challenge to the South African fruit growing industry is to establish an alternative, non-chemical approach to control stem canker fungi. Such an assignment deals with Diaporthe ambigua as a canker pathogen of pome and stone fruit trees in South Africa. Special emphasis is given to the possible development of a biological control strategy. Endeavors included the search for hypovirulence, a reduced virulence associated with the presence of doublestranded RNA (dsRNA). The introductory section of this chapter presents a comprehensive review of the literature pertaining to those Diaporthe spp. directly or indirectly implicated with canker of pome, stone, and woody small fruit trees. Special emphasis is given to the disease situation in South Africa. Over-reliance on host association has led to the description of a plethora of Diaporthe spp. on a wide range of hosts. D. ambigua is regarded as one of several small-spored species associated with Diaporthe canker of pome, stone, and woody small fruit trees, and their rootstocks worldwide. Results presented in a subsequent section attribute a newly recognized canker disease of apple, pear, and plum rootstocks in South Africa to a species of Diaporthe. Supported by morphological data and random amplified polymorphic DNA analysis of South African and reference strains of Diaporthe, our research has shown that D. ambigua is the most appropriate name for the fungus. Nursery rootstocks infected with D. ambigua are readily killed by this fungus. This is in contrast to mature rootstocks which are normally killed over an extended period of time. All deciduous fruit cultivars that we have tested were susceptible to D. ambigua infection. A significant strain x rootstock interaction was found, indicating that strains of D. ambigua react differently on various rootstocks. The population of D. ambigua in apple, pear, and plum orchards in South Africa represents a large number of vegetative compatibility groups (VCGs) that occur in close proximity to one another. Results suggest that ascospores act as the primary propagules of infection. Increasing numbers of VCGs in populations lacking predominance of one VCG, as found in D. ambigua, may reduce the effectiveness of dsRNA transfer and thus the effectiveness of biological control. However, all dsRNA-containing strains of D. ambigua originated from ascospores representative of a spectrum of VCGs, suggesting transfer of dsRNA to sexual progeny. Based on its association with ascospores, it is possible that the relatively small dsRNA element in D. ambigua is associated with mitochondria. Interest in the viral dsRNA associated with hypovirulence of D. ambigua stemmed primarily from its potential as a biological control agent. Strains of this fungus harboring the single dsRNA segment, although able to initiate an infection, generally cause only limited invasion. Results presented in a following section demonstrate the conversion of vegetatively compatible virulent strains of D. ambigua to the hypovirulence phenotype following transmission of the hypovirulence-associated dsRNA via anastomosis.