X-M XU, AM BERRIE and DC HARRIS

Horticulture Research International, East Mailing, West Mailing, Kent ME19 6BJ, UK

Background and objectives
Brown rot of apple and pear, caused by Monilinia frunctigena, causes serious losses every year both in the orchard and in store [11. All cultivars are susceptible. Losses in the orchard are less well quantified and past observations indicated that for every fruit developing brown rot in store several are lost in the field. Currently in the UK, routine post-harvest drenching reduces storage losses by about half. A research programme is being conducted at HRI, East Mailing to elucidate the epidemiology of brown rot for development of a more rational control strategy.

Materials and methods
Two orchards (one apple and one pear) were observed twice weekly from July to harvest for brown rots and the origin of every rot was identified. A Burkard spore trap was used to trap conidia. Inoculation of potted apple trees was conducted in a glasshouse to investigate the effects of wound age and fruit maturity on incidence of brown rot.

Results and conciusions
Observations confirm that mummified fruits of pear and apple on the ground and remaining attached to the tree are important sources of overwintering inoculum. Overwintering spur cankers due to M. fructigena were also observed but less frequently than mummified fruits. Conidia of M. fructigena were detected in a pear orchard during two periods in growing season: low numbers were recorded sporadically during about two weeks in June and low to moderate numbers obtained from late July to late September when recording was discontinued. In an apple orchard, conidia were detected only during the second of these periods.

Brown rot on apple and pear trees first appeared in early-July and increased markedly from late August to harvest. Most of the rots observed on apple and pear resulted from primary infection, all associated with damage caused by birds, insects or growth cracking. The relative importance of these entry sites depended on cultivar, orchard and season. On cv Cox, most of the primary rots were associated with bird damage in 1996; in contrast, in 1997, the proportion of the primary rots associated with growth cracking was higher because of frost injury and unusually high temperatures in the early summer. Brown rot in cv Bramley occurred earlier than in cv Cox. In cv Bramley, earlier rots associated with growth cracking resulted from frost injury or herbicide injury and later originated from bird and insect damage. For pears, most of the primary rots were due to bird damage in both years. More secondary (contact spread) rots were recorded on pear than on apple.

Inoculation of fruits on potted apple trees (cv Cox and Gala) showed that wounding was essential for infection by M. fructigena. The spread from rotting fruit to the subtending spur/shoot occurred within 4 days. The incidence of brown rot depended on the interaction between fruit maturity and wound age: wounds on younger fruits were more resistant to infection than on older fruits and, the older the wound the more resistant it was to infection. Furthermore, the degree of wound age-related resistance was higher on younger fruits than on older fruits. The combined effects of weather, wound age and fruit maturity are currently being investigated.

References
1. Byrde RJW, Willetts HJ, 1977. The brown rot fungi of fruit: their bioiogy and control. Pergamon Press, Oxford UK.