Studies of the biology and control of the grey mould fungus of pipfruit: 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

Growth of the grey mould fungus, Botrytis cinerea, a storage rot of apples and pears was investigated both in culture and in inoculation studies using 3 apple cultivars and 1 pear cultivar. Control of the fungus with fungicides was also investigated. On potato dextrose agar, mycelial growth was fastest at 18°, an 85 mm diameter plate being covered in 4 days at this temperature compared with 7 days at 25°, 9 days at 10° and 18 days at 4°. The fungus did not grow as well at 25° as at 18° and although initially the growth rate was the same as at 18°, after 3 days the fungus suddenly slowed to one half the initial rate. Greyish-brown coloured conidia were formed at all temperatures once the fungus had covered the plate, but sporulation did not occur when cultures were incubated in the dark. Black, irregularly-shaped sclerotia, 1.5-6 mm x 1-3 mm in size, were produced in culture. At 25° many more sclerotia were formed, but they were less than half the size of those formed at 4°, 10° and 18°. In pathogenicity studies using Granny Smith and Splendour apples and Packham's Triumph pears, the fungus was found to grow best on Splendour apples. Rotting developed 1.5 times faster on this cultivar than on Granny Smith or Packham's Triumph, when the rate of rotting was similar. The stage of maturity at harvest had only a minor effect on rotting. Rotting developed 10% faster on Granny Smith and Splendour apples harvested on 12 May, than on apples harvested 4 weeks earlier. Apples harvested on 12 May had a flesh firmness of 7.6 kg (Penetrometer reading) and a soluble solids level of 13.5% (Refractometer reading) compared with 8.3 kg and 12% for fruit harvested later. Conidia were not formed on rotted apples except where they were cut open and the flesh exposed but sporulation occurred on intact rotted pears. Both iprodione and imazalil inhibited mycelial growth of B. cinerea at 100 ppm a.i. in vitro. With benomyl a slight amount of growth was observed at 100 ppm a.i., but at 500 ppm mycelial growth was completely inhibited. Captan did not inhibit mycelial growth except at 2,500 ppm a. i. Iprodione and benomyl were tested at 500 ppm a.i. on Granny Smith apples wounded in 2 places and inoculated by dipping in a spore suspension of the fungus (2.88 x 10 5 spores/ml. Both fungicides preventing rotting when applied 4 h after inoculation, but a delay of 24 h resulted in a high incidence of decay. Iprodione was the more effective chemical when applied 24 h after inoculation, reducing decay by 50% compared with 10% for benomyl. The fungus was unable to infect intact apples even in a saturated atmosphere with free moisture present. Penetration and infection only occurred through wounds.