Most of the rots will be missed if condition checking is too early.

Kiwifruit become infected with Botrytis at harvest, but it takes between 4 and 12 weeks of coolstorage before the rot symptoms appear. If condition checking is carried out too early, many Botrytis infections will go undetected because they are still invisible. They will continue to develop after the fruit has been repacked and returned to the coolstore.

Condition checking can be an effective tool for dealing with Botrytis rots. However, if it is carried out at too early a stage of coolstorage, it will fail to identify a large proportion of the infected fruit - additional Botrytis rots will appear almost as soon as the condition checking is completed. Early condition checking can identify lines of fruit with major Botrytis problems, but repacking needs to be delayed until a later stage of coolstorage.

Condition checking relies on visible rot symptoms to identify Botrytis-infected fruit, and rot symptoms first become visible over the same period of time ( see figure 1) in all lines of kiwifruit coolstored at 0°C:

• the earliest symptoms of primary Botrytis infections (the infections that occur through the picking wound at harvest) begin to appear after 4-5 weeks of coolstorage.

• over the next few weeks, there is a rapid increase, and then an equally rapid decrease, in the numbers of additional rots that appear.

• after 10-12 weeks of coolstorage, most of the primary Botrytis rots have become visible.

• after that, very few, if any, additional primary rots will appear, but the incidence of secondary rots ("nesting", or the spread of Botrytis from fruit to fruit within the pack) will continue to increase.

(N.B. This "timetable" applies only to fruit held at 0°C; Botrytis rots begin to appear after as little as 2-3 weeks in fruit that are held a degree or two above zero.)

Figure 1: Incidence of new primary Botrytis rots (stem-end rots) in a line of coolstored kiwifruit (1994 harvest; Kerikeri).

Implications

Another way of recording the onset of Botrytis symptoms is to add up a running total as each batch of new rots is observed; Figure 2shows the cumulative rots from 6 different lines of kiwifruit which had final levels of primary Botrytis rots between 2% and 45%. This wide variation in total rots tends to obscure any similarities between the different curves in Figure 2. However, if the data are expressed as percentages of the final level of rots for each curve, they all follow a similar pattern (Figure 3). This is true not just for these 6 lines of fruit but for all the kiwifruit we have assessed for Botrytis rots during the past 14 years.

Figure 2: Cumulative incidence of primary Botrytis rots (stem-end rots) in 6 lines of coolstored kiwifruit (1994 harvest; Bay of Plenty, South Auckland, and Kerikeri).

Figure 3: Cumulative incidence of primary Botrytis rots (stem-end rots) in 6 lines of coolstored kiwifruit, expressed as percentages of the final level of primary Botrytis rots. Data are from the same 6 lines of fruit as in Figure 2.

What are the implications for condition checking?

The optimum time to carry out condition checking for Botrytis is after 10-12 weeks of coolstorage:

• almost all the primary infections will have produced visible symptoms, which enable the affected fruit to be identified and removed.

• not much nesting will have developed.

Condition checking after more than 12 weeks of coolstorage will also enable the removal of all Botrytis-infected fruit, but losses from nesting will increase as condition checking is delayed, and rots caused by other fungi will begin to appear.

However, if condition checking is carried out before 10 weeks of coolstorage, some of the primary Botrytis infections will still be invisible and therefore undetectable. These undetected primary infections will continue to develop into rots (and to initiate nesting) after the condition checked pallets have been repacked and returned to the coolstore. The earlier repacking is carried out, the bigger this problem will be (Figure 4).

Figure 4: Proportions of the final total incidence of primary Botrytis rots of kiwifruit that will be detected/undetected by condition checking after different periods of coolstorage.

Tool

Condition checking after 8 weeks' coolstorage will fail to detect approximately 10% of the Botrytis infections (Figure 4). In lines of fruit with low levels of Botrytis infection this 10% may represent an acceptably small number of rots retained in the packs, but failure to detect 10% of rots in lines with high levels of infection may be misinterpreted as a failure of the NZKMB's condition checking protocol itself.

Condition checking after only 6 weeks coolstorage will fail to detect approximately 80% of the infections (Figure 4). There are only limited benefits from condition checking for Botrytis this early:

• lines of fruit that fail condition checking at 6 weeks will be identified as having a major Botrytis problem, but they should not be repacked until some weeks later when all the infections have developed into visible rots.

However:

• lines of fruit that "pass" condition checking at 6 weeks are not necessarily low Botrytis lines; they may in fact be time-bombs that will develop unacceptable levels of rots within one week or less!

Condition checking is a powerful tool for dealing with Botrytis rots. However, its role changes at different stages of coolstorage. Early on, it can identify bad Botrytis lines, but only after 10 weeks of coolstorage does it become a comprehensive method of dealing with Botrytis rots.