Studies of the biology and pathogenicity of the black rot fungus on apple: A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Horticultural Science with Honours in the University of Canterbury

Abstract

The growth and pathogenicity of the black rot fungus Botryosphaeria obtusa (Schw.) Shoemaker, were investigated using agar plate and apple inoculation experiments. Mycelial growth was faster and more sporulation occurred on potato dextrose agar with yeast extract (Marmite) added (PMDA), than on potato dextrose agar (PDA) or on corn meal agar (CMA) at 10°, 25° and 30°. At 20°, the culture on PMDA measured 8 cm in diameter after 5 days incubation, whereas the colonies on PDA and CMA had a diameter of 7.3 cm and 1.2 cm respectively. The growth rate was most rapid at 30°. Colony diameter increased most rapidly on all media tested at 30° but the maximum size of the colonies at this temperature was not as great as at 20° and 25°. At 30° growth was raised but at 20° the mycelium was closely appressed to the medium. At 30° the hyphae were thicker and darkened rapidly, there were many short.branches and when transferred to a slide for microscopical examination the hyphae were easily broken. Growth on all media was slow at 4° and 10°; after five days incubation, the colonies on PMDA were 0.2 cm in diameter at 4° and 2.5 cm at 100. There was little difference in the rate of rot development when fruit was inoculated using either a cork borer or a slit method but the cork borer method was found to give more consistent results as the rot tended to increase radially from the point of inoculation. Rotting was assessed by three methods : (1) area infected, using standard area diagrams; (2) external length and width of the rot on the surface of the fruit; (3) depth of penetration of the rot into the apple tissue. Assessment using standard area diagrams was found to be most consistent as both the length and width of the rot was considered. Different amounts of rot were found on each of the apple cultivars tested. They could be placed into three groups according to the rate of rotting and amount of rot produced. Cox's Orange and Gala apples rotted rapidly; less rapid rotting occurred on Golden Delicious, Red Delicious and Splendour; and slowest rotting was on Granny Smith and Stunner apples. In one inoculation experiment at 20° which terminated at the end of March, 16 days after inoculation, the rotted area was 100% for Gala and Cox's Orange, 90% for Red Delicious, 80% for Splendour and Golden Delicious, 60% for Granny Smith and 40% for Sturmer. Similar trends were observed for the other experiments at 20° and 10° which were inoculated when the fruit was more mature. On immature fruit, the rot ceased to develop earlier compared with mature fruit and the area of rot was smaller on immature fruits at all incubation temperatures. Sturmer apples held at 20°c and inoculated in March, April, May and August developed 43%, 51%, 78% and 98% rot respectively, five weeks after inoculation showing the effects of increasing maturity. Fruit incubated at 20° rotted rapidly compared with that held at 4° and 10°. In an experiment initiated in April maximum ,rot and pycnid1al formation occured on the seyen apple cultivars at 20°, while the same cultivars stored at 10° rotted slower and pycnidia were formed later. Maximum rot occurred after three weeks on fruit incubated at 20°, but not until 10 weeks for apples stored at 10°. Granny Smith and Sturmer apples inoculated in May and stored at 4° had 18.5% of their area rotted 23 weeks after inoculation. The fungus remained viable in these apples as , when they were transferred to 20°, the rot developed rapidly and pycnidia were formed.