Researchers find close links between Botrytis spores on dead leaves and rots in the coolstore.

Results of field trials show that the incidence of storage rots is related to the amount of Botrytis detected in the field prior to harvest. A higher incidence of Botrytis on necrotic leaves, provides inoculum available for infection of fruit, with a subsequent increase in the incidence of storage rots. We can thus predict the incidence of rots likely to develop in storage before the fruit is picked. Incubating dead leaf tissue to determine Botrytis incidence in the field provides an excellent prediction of storage rots.

Botrytis storage rot is one of the most important disease problems facing the kiwifruit industry. In addition to direct and indirect grower losses due to infected fruit there are two further serious implications for fruit quality:

• ethylene produced by rotting fruit prior to the appearance of visible symptoms can lead to softening of other fruit in the tray resulting in reduced storage life.

• fruit are often exported before the first rots appear which may necessitate repacking overseas to remove both rotted and soft fruit which have developed during transit.

The ability to predict the likely incidence of storage rots in a particular line of fruit while on the vine would be a major asset for marketing and management of stored fruit. Several post-harvest methods for predicting Botrytis stem-end rot in storage have been reported to the NZKMB. Shaun Pennycook developed a method of withdrawing a sample of fruit from a grower's line and accelerating the development of rots. Nick Pyke also looked at accelerating rots and attempted to assess the presence of Botrytis using agar plugs dabbed on to the picking wounds. We report here on a prediction system we have recently developed based on an assessment of the levels of Botrytis inoculum in the orchard.

It is known that lines of fruit from different orchards, have different incidences of Botrytis rots in storage. Recent studies undertaken for the NZKMB have shown that this relates to differences in growing conditions in different orchards, to the amount of Botrytis inoculum present at harvest and also to differences in the ways which the fruit is handled after harvest. Although stem end rots are first observed after four to six weeks in coolstorage, it has been shown that infection actually occurs at harvest time through the picking wound.

Ten plots (each of two pergola bays) spread over a 136 vine block were used to investigate the relationship between the level of Botrytis in a South Auckland kiwifruit orchard during the preharvest period and the number of Botrytis stem-end rots which developed in storage. Regular preharvest assessments of the incidence of Botrytis on necrotic leaf tissue were made by incubating samples of this tissue and checking for the presence of Botrytis sporulation. Ten discs of necrotic leaf tissue were taken from each plot in the orchard block at each of nine sampling dates. Samples were incubated for 20 hours at 14°C and high humidity then assessed for the presence of Botrytis. These assessments provide an indication of inoculum available to infect the picking wound.

Figure 1: Prediction of storage rots developing after 10 weeks storage from the combined incidence of Botrytis on leaf discs over the entire monitoring period (April 7 to May 13, 1992). Values are the mean of each plot from a "high" disease risk block. Fruit were direct picked into trays.

The relationship between the incidence of stem-end rots after 10 weeks storage and the total number of Botrytis colonies recovered from leaf discs over a six-week period prior to harvest (Figure 1) indicates that the incidence of storage rots is determined by the amount of inoculum present in the orchard prior to harvest. Spores are produced on the necrotic leaf tissue and accumulate on fruit surface. The relationship remains the same if the incidence of Botrytis on leaf discs of two postharvest assessments over a one-week period is substituted for the preharvest assessment (Figure 2). This agrees with our previous findings that dead leaves are the main source of inoculum in the field.

Figure 2: Prediction of storage rots developing after 10 weeks storage from the incidence of Botrytis on leaf discs on four assessments taken over a 1 week period after harvest. Values are the mean of each plot from the "high" disease block. Plots with no Botrytis have been omitted from the regression. Fruit were direct picked into trays.

Does this relationship apply to other orchards and other localities? A very good relationship was obtained between the number of Botrytis stem-end rots at harvest and the number of necrotic leaf discs with Botrytis (Figure 3) when data from this trial was combined with data from similar trials done in Te Puke. Leaf discs sampling was carried out at one date just prior to fruit harvest in the Te Puke trials. In each of these trials the fruit were subjected to either jostling or grading after harvest which transfers spores from fruit surfaces to the picking scar. The high accuracy (R² = 0.98) of the predictions is due to the large sample size used (a minimum of 50 discs and 30 trays of fruit).

Figure 3: Prediction of storage rots developing after 10 weeks storage from the incidence of Botrytis on leaf discs at harvest. Each point represents the mean incidence on at least 50 leaf discs and in 30 trays of fruit. The trials include one based at South Auckland and four at Te Puke. Fruit were subjected to "handling" (eg Jostling or grading) after harvest.)