What is it ?

Figure 1: Blowouts are a common source of Botrytis infection.

Botrytis causes several kinds of rots in kiwifruit. Three kinds of Botrytis rots can develop in coolstorage. The most common are stem-end rots which develop in coolstore at 0ºC from Botrytis contamination of the picking wound at, or soon after, harvest. Secondary rotting is caused when the fungus spreads from rotten fruit to adjacent fruit in the tray or bin. When fruit become over-mature, Botrytis is one of several fungi that cause breakdown rots. (Brook, 1990). A single fruit infected by Botrytis can also accelerate premature softening of the remaining fruit in a tray as a result of the ethylene produced during Botrytis fruit infection.

What does it look like ?

As the name suggests, stem end rotting begins at the stem end of undamaged kiwifruit and progresses down the fruit. This results in the classic tide mark on the outside of the fruit. Inside the diseased flesh is glassy and water-soaked. Secondary rotting results in the complete breakdown of the fruit.

Figure 2: Botrytis spores on stored kiwifruit	Figure 3: Secondary Botrytis rots, or ‘nesting’ in a tray of kiwifruit

What is the disease life-cycle ?

Botrytis survives as dormant mycelium or small, hard, black sclerotia over the winter. In the spring, germination takes place to produce conidia (asexual spores) which are dispersed by wind. Early on in the growing season, most of the spores are produced from mulched prunings on the ground (Elmer et al., 1993) or from Botrytis sources outside the block (eg. glasshouse vegetables). By petal fall, Botrytis populations can increase dramatically to up to 9600 million spores which coincides with abundant dead and dying flower parts. This build-up may occur on the males before being dispersed to the females. Midway through the growing season Botrytis is found on senescent petals attached to fruit and blowouts. By harvest, the green leaves with necrosis (dead patches) and dead leaves are the primary source of Botrytis inoculum. Research has shown that the hairy kiwifruit acts as a natural spore trap and thousands of Botrytis spores have been measured on the fruit surface. At the time of harvest, spores on the skin surface contaminate the picking scar which leads to stem end rot in coolstorage. After harvest our knowledge of the Botrytis life-cycle is less clear.

Figure 4: Botrytis spores growing on a pruned branch on the ground
Figure 5: Botrytis on a male flower	Figure 6: An infected detached flower petal has attached to this green leaf causing the Botyrtis infection to spread to healthy leaf tissue

What conditions are needed ?

Moisture is essential for spore germination, infection, growth and sporulation of Botrytis. The optimum temperature for development is about 18ºC. Botrytis levels increase when either air movement or sunlight are restricted within the orchard. For this reason fruit grown under dense canopies are more likely to develop Botrytis than those under canopies which allow good light penetration. Fruit grown in blocks with high spore numbers are likely to be at more risk from Botrytis in storage.

Where is it found ?

Botrytis of kiwifruit is found in all the growing regions around New Zealand but the incidence varies from orchard to orchard, from region to region, and from year to year. Botrytis also causes significant losses in California, Italy, Greece and Chile.

Why is it a problem ?

Although infection occurs at harvest, disease symptoms do not usually show up until between 4-12 weeks of coolstorage at 0ºC (or sooner if the fruit is stored at higher temperatures). Fruit which develop Botrytis rots are unsuitable for sale. Often costly repacking in the offshore market is necessary. As well as causing rotting, Botrytis in a tray of fruit causes higher levels of ethylene gas to be released and this accelerates fruit softening.

What are the current chemical controls ?

a) Kiwigreen orchards. Dicarboximide sprays at harvest were the traditional method of chemical control of Botrytis. However, Botrytis has developed resistance to the dicarboximides such as Ronilan and Rovral. Some strains of Botrytis are now also resistant to the benzimidazoles (Benlate). Therefore such chemicals must be used sparingly and other methods of Botrytis control are recommended.

b) Organic orchards. Growers do not apply fungicides for Botrytis and industry data suggests that stem-end rot losses are lower. Research is underway to investigate why organic fruit have less Botrytis problems.

Are there any other forms of control ?

Control starts on the orchard. As mentioned above, an open canopy, plenty of air movement in the block and good orchard hygiene to remove sources of Botrytis spores such as blowouts, all go a long way to reducing the risk of Botrytis on your orchard.

Postharvest handling of the fruit (picking, grading and packing) allows the transfer of spores from the surface of one fruit onto the picking wounds of adjacent fruit, resulting in a potential infection. Minimising fruit handling will reduce spore transfer.

The inside of canvas picking bags can become contaminated with Botrytis spores attached to kiwifruit hairs. Research has shown that fishing net type bags may reduce Botrytis build-up in the bag.

Holding or "curing" the fruit at ambient temperatures for at least 2 days before packing and coolstorage significantly decreases Botrytis storage rots. Rapid cooling of fruit may increase Botrytis stem-end rot (Lallu et al., 1992).

Fruit need to be condition checked in coolstore. The optimum time for condition checking is after 10-12 weeks of coolstorage, by which time almost all the primary infections will have produced visible symptoms. Early condition checking eg. after 6 weeks storage will identify any major problems but will fail to detect approximately 80% of the infections (Manning et al., 1995).

Biological control agents (BCAs) appear very promising for the control of Botrytis. Work is currently underway to identify and test naturally occurring effective isolates of antagonist fungi which can suppress Botrytis in the field. Postharvest applications of natural products such as 6PAP have also successfully at suppressed Botrytis storage rots to commercially acceptable levels. Research to identify the most effective method of applying 6PAP to the picking wound is in progress.

Zespri International in conjunction with HortResearch scientists have developed a Botrytis management programme to enable postharvest operators to predict the at-risk lines of fruit and manage the handling of such lines accordingly.

References

Brook, P.J. 1990. Review :Botrytis cinerea and fruit rot of kiwifruit, Actinidia deliciosa. Report to the Scientific Research Committee, New Zealand Kiwifruit Marketing Board, DSIR publication.

Elmer, P.A.G. et al. 1994. Sources of Botrytis in vines and spores on the fruit surface at harvest. National Research Conference 1994, NZ Kiwifruit Marketing Board : 19-21.

Elmer, P.A.G. et al. 1993. Epidemiology of Botrytis cinerea in Motueka kiwifruit orchards. Report to New Zealand Kiwifruit Marketing Board for NZKMB project # 1992/58. HortResearch Client Report No. 93/249.

Lallu, N., H.J. Elgar, C.W. Yearsley, and M. Manning. 1992. Postharvest handling affects the incidence of Botrytis. National Research Conference 1992, NZ Kiwifruit Marketing Board : 24-25.

Manning, M.A., H.A. Pak, and S.R. Pennycook. 1995. Timing condition checking to catch Botrytis rots. NZ Kiwifruit Journal, February/ March 1995 : 24 -25.

Manning, M.A. and H.A. Pak 1993. New insights into Botrytis. NZ Kiwifruit Journal, April/ May 1993 :15-18.

Pak H.A. and M.A. Manning. 1995. How does spraying for Sclerotinia in kiwifruit affect Botrytis ? NZ Kiwifruit Journal, October 1995 : 19-20.

Pak H.A. et al. 1992. Epidemiology of Botrytis on kiwifruit. Report to the NZ Kiwifruit Marketing Board. HortResearch Client Report No. 92/17.