| Summary: | Thesis (MScAgric (Plant Pathology))--University of Stellenbosch, 2010. === ENGLISH ABSTRACT: A survey was undertaken on apple and pear trees in the Western Cape Province to determine
the aetiology of trunk diseases with reference to trunk diseases occurring on grapevine.
Grapevine trunk diseases cause the gradual decline and dieback of vines resulting in a
decrease in the vine’s capability to carry and ripen fruit. In recent years, viticulture has been
expanding into several of the well established pome fruit growing areas. The presence of
trunk pathogens in pome fruit orchards may affect the health of the pome fruit trees as well as
cause a threat to young vineyards planted in close proximity to these potential sources of
viable inoculum.
Several genera containing species known to be involved in trunk disease on pome
fruit and grapevine were found, including Diplodia, Neofusicoccum, Eutypa,
Phaeoacremonium and Phomopsis. Diplodia seriata and D. pyricolum, were isolated along
with N. australe and N. vitifusiforme. Four Phaeoacremonium species, P. aleophilum, P.
iranianum, P. mortoniae and P. viticola, two Phomopsis species linked to clades identified in
former studies as Phomopsis sp. 1 and Phomopsis sp. 7, and Eutypa lata were found. In
addition, Paraconiothyrium brasiliense and Pa. variabile, and an unidentified Pyrenochaetalike
species were found. Of these the Phaeoacremonium species have not been found on pear
wood and it is a first report of P. aleophilum occurring on apple. This is also a first report of
the Phomopsis species and Eutypa lata found occurring on pome trees in South Africa
Two new coelomycetous fungi were also found including a Diplodia species,
Diplodia pyricolum sp. nov., and a new genus, Pyrenochaetoides gen. nov. with the type
species, Pyrenochaetoides mali sp. nov., were described from necrotic pear and apple wood.
The combined ITS and EF1-α phylogeny supported the new Diplodia species, which is
closely related to D. mutila and D. africana. The new species is characterised by conidia that
become pigmented and 1-septate within the pycnidium, and that are intermediate in size
between the latter two Diplodia species. Phylogenetic inference of the SSU of the unknown
coelomycete provided bootstrap support (100%) for a monophyletic clade unrelated to known
genera, and basal to Phoma and its relatives. Morphologically the new genus is characterised
by pycnidial with elongated necks that lack setae, cylindrical conidiophores that are seldomly
branched at the base, and Phoma-like conidia. The phylogenetic results combined with its
dissimilarity from genera allied to Phoma, lead to the conclusion that this species represents a
new genus. A pathogenicity trial was undertaken to examine the role of these species on apple,
pear and grapevine shoots. N. australe caused the longest lesions on grapevine shoots, while
Pyrenochaetoides mali, Pa. variabile, D. seriata and P. mortoniae caused lesions that were
significantly longer than the control inoculations. On pears, D. pyricolum and N. australe
caused the longest lesions, followed by D. seriata and E. lata. On apples, the longest lesions
were caused by N. australe and P. iranianum. D. seriata, D. pyricolum, E. lata, N.
vitifusiforme, Pa. brasiliense, P. aleophilum and P. mortoniae also caused lesions on apple
that were significantly longer than the control.
The study demonstrated that close cultivation of grapevine to apple and pear orchards
may have inherent risks in terms of the free availability of viable inoculum of trunk disease
pathogens. === No Afrikaans abstract available.
|
|---|
