What is it ?

The fungus, Sclerotinia sclerotiorum, infects kiwifruit blossoms at flowering causing severe fruit losses in some orchards and some years. Over 400 plant species act as hosts for the Sclerotinia fungus.

What does it look like ?

There are three distinct phases to the disease. The first sign of Sclerotinia is often the infection of male blossoms. Sometimes you can see white mycelia on the rotted flower masses. The second phase is infection on female vines during, or soon after, petal fall. The developing fruitlet and its stalk quickly rot and turn light brown. The fruitlets usually dry up and remain attached to the vine for several weeks or months.

Fig 1: Diseased blossom hanging in vine (January)	Fig 2: Slight (1cm) scarring on fruit in orchard	Fig 3: Severe scarring on fruit in orchard

The third stage is field rotting which can occur during December, January and February. The rot initially appears as a small pale green, soft lesion on the fruit surface, often associated with flower parts attached to the shoulder, side or basal areas of the fruit. Under continuously wet conditions, the rot will progress and cause the fruit to drop. A change in environmental conditions, and/or the plant’s natural defence mechanisms, can halt the progress of the rot resulting in scarring of the fruit surface. Scars range from superficial (3-4 mm across) to severe (up to 20 - 40 mm in length).

What is the disease life-cycle ?

Sclerotinia overwinters in the soil as special structures called sclerotia. Sclerotia are hard, black bodies which form on infected blossoms and rotten fruit. In the spring tiny mushrooms called apothecia emerge from the sclerotia and release spores (ascospores). Ascospores are the primary inoculum for all Sclerotinia infections and disease. Spores are present on most days from late October to early May, although ascospore numbers appear to fluctuate from day to day. The ascospores land on kiwifruit flower petals and if conditions are favourable (warm and wet) infection will occur. By autumn the cycle starts over again.

Fig 4: Diseased fruit and blossoms on orchard floor	Fig 5: Sclerotia on diseased fruit

What conditions are needed ?

Warm, wet weather is conducive to infection during flowering and later in the season. Sclerotinia ascospores can not directly infect healthy green tissues such as leaves and fruit but will readily infect senescing or dying tissues during petal fall, such as petals or stamens. Once the fungus is established in this tissue, it is then able to penetrate and infect healthy tissue. We still do not fully understand the precise environmental conditions required for infection, the time required for infection, whether or not ascospores can directly infect immature kiwifruit, and how long Sclerotinia can survive within floral tissues.

Where is it found ?

Sclerotinia is found in all kiwifruit and vegetable growing regions in New Zealand and in all major crop producing regions around the world.

Why is it a problem ?

Many orchardists suffer high levels of fruit loss as a result of Sclerotinia infection, either at flowering or due to rots or scarring of the fruit. Assessment of disease loss can be difficult to evaluate because : a) fruitlets and fruit drop to the ground, and b) infections occur over 3-4 months. We still do not know enough about the disease to implement an effective disease management programme.

What are the current chemical controls ?

Fungicides (Benlate, Rovral or Ronilan) should be applied mid-flowering to petal fall, which is when the floral tissues are at greatest risk from infection. Unfortunately, Botrytis has developed resistance to the fungicides used for Sclerotinia control (dicarboximides and benzimidazoles). To prevent further resistant strains of Botrytis developing, only spray for Sclerotinia if necessary - that is if the orchard has had a history of the disease and wet weather is forecast over flowering. Be careful to integrate your Botrytis and Sclerotinia spray management programmes.

Are there any other forms of control ?

Minimise the chance of infection, for example by maintaining relatively open shelter and vine canopies in an attempt to keep moisture levels low within the canopy.

HortResearch scientists are developing a disease risk prediction model for Sclerotinia which will incorporate data on previous disease records, inoculum levels, crop growth stage and environmental conditions to identify the need for sprays and the most effective time of application.

The scientists are also investigating biological control agents, including naturally occurring fungi, bacteria and yeasts which suppress the Sclerotinia fungus as natural alternatives to chemical fungicides.

References

Hoyte, S. 1996. Does Sclerotinia infect your floral tissue ? New Zealand Kiwifruit Journal, November 1996 : 11-12.

Hoyte, S. 1994. Sclerotinia and its control. New Zealand Kiwifruit Marketing Board National Research Conference, 1994 : 18-19.

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