No abstract

Experiments were conducted to investigate the effects of temperature and relative humidity (RH) on the in vitro germination and viability of conidia of the apple brown rot fungus (Monilinia fructigena), and on colonization and sporulation on detached fruits by M. fructigena. Conidia only germinated under near-saturation humidity ($ 97% RH) and the rate of germination initially increased with temperature to a maximum at < 23±258C and then decreased. Conidia germinated rapidly ± more than 70% of viable conidia had germinated within 2 h at 20 and 258C. The rate of colonization on detached fruits increased log-linearly with increasing temperature. Sporulation on detached fruits was not observed at 5 or 258C; sporulation appeared to be unaffected by either temperature (10± 208C) or RH (45±98%) once infection was established. Detached conidia remained viable for a long period of time, up to 20 days, the longest assessment time in this study, depending on storage temperature (10 or 208C) and RH (45 or 85%). Temperature appeared to be more important than RH in affecting conidial viability. Low temperature and high RH resulted in reduced loss of conidial viability. Storage at 108C and 85% RH for up to 20 days appeared not to affect conidial viability. These results indicate that environmental conditions during the main UK growing seasons are unlikely to be limiting factors for the development of brown rot on apple.

Effects of fruit maturity, wound age, temperature and the duration of wetness periods on infection of apple fruits by conidia of the brown rot fungus, Monilinia fructigena, were studied. Inoculation of fruits on potted apple trees and harvested mature fruits showed that wounding was essential for infection by M. fructigena. On potted trees, there was a significant difference between the susceptibility of cvs Cox and Gala and this difference depended on wound age. The incidence of brown rot was affected greatly by fruit maturity and wound age. Wounds on younger fruits were more resistant to infection than those on older fruits, whilst the older the wound, the more resistant it was to infection. Furthermore, the degree of wound age-related resistance was greater on younger fruits than on older fruits. These relationships were well described by regression models. The effect of the duration of wetness periods was very small: increasing the duration of wetness periods reduced the incidence of brown rot on older wounds. For detached fruits, all those wounded were rotted after inoculation, except for those in two treatments under 20ЊC on fruits with wounds which were 8 days old. The incubation period of the fungus was generally very short. Wound age was the single most important factor influencing the length of the incubation period; the incubation period increased as wound age increased.

Spatio-temporal development of brown rot (Monilinia fructigena) on apple and pear was monitored in an apple (cv. Cox) orchard and a pear orchard of several cultivars over several years. Disease on individual trees was recorded weekly from July to harvest, individual fruits with brown rot were tagged but not removed and rot-origin identified. On apple cv. Cox and pear (cvs Conference and Comice), all primary rot arose from infection via wounds caused by insects, birds and growth cracks. Birds were the most important wounding agents on pear in the field. Secondary (fruit-to-fruit contact) rot was considerably less than primary rot, especially for pear. Incidence of disease (percentage of fruits with brown rot) increased gradually from late July up to harvest; the final disease incidence varied with seasons and cultivars, ranging from 1 to 11%. For pear, Comice had greater incidence than Conference. Significant aggregation of diseased fruits among trees was detected for assessment dates when the overall incidence of disease was greater than 0´5%. On Cox and Conference, significant correlation of disease incidence between adjacent trees or trees separated by one or more trees (i.e. spatial lag measured as units of distance between adjacent trees) was detected, but there was no clear relationship between the correlation, the distance or time. For Comice, there was consistent and significant positive correlation of brown rot incidence over 3 years. It is speculated that behavioural characteristics of wounding agents may have played an important role in influencing the spatio-temporal dynamics of brown rot on apple and pear.

Neonectria galligena can cause European canker of apple as well as fruit rot. Healthy unwounded fruits on potted trees of cvs Cox, Bramley and Gala were inoculated with conidia of N. galligena to investigate the effects of wetness duration and fruit maturity on rot development. Overall, the incidence of fruit rot was influenced more by fruit maturity at the time of inoculation than by duration of wetness (6-48 h). Young fruit were most susceptible to infection, with 50% of fruit infected when inoculated up to 4 weeks after full bloom. The susceptibility decreased initially until c. 2 months after full bloom and then increased gradually until harvest. Almost all preharvest symptoms (eye rot) developed only on the fruit inoculated up to 4 weeks after full bloom. All other rots were observed after six-month postharvest storage under controlled atmospheric conditions. However, the relative proportion of preharvest eye rots and postharvest storage rots varied greatly among three years. The effect of wetness duration was only significant for fruit inoculated in their early stages of development but not for those inoculated near harvest. Regression models were developed to describe the observed effects of fruit maturity and wetness on the incidence of total nectria rots.