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Scientific name
Phomopsis castanea
Introduction
Phomopsis nut rot, caused by Phomopsis castanea, is the most important post-harvest disease of chestnut in Australia. It is present in the major production areas including Victoria, New South Wales, South Australia and Western Australia and has been found to affect all cultivars of chestnut tested to date. Phomopsis nut rot can cause significant losses; up to 85% of nuts were diseased in some large consignments of nuts in the market. Because rotten nuts may appear healthy on the outside, special measures are required to ensure consumers buy only good quality nuts.
Symptoms
Chestnuts with advanced kernel rot may have a dark shell, are soft to touch and appear mouldy. Badly-rotted nut shells may be covered with small-raised, cream-coloured structures which are the fruiting bodies of the causal fungus. Sometimes, these symptoms appear on nuts still in their burrs on the tree, but they are mostly found in consignments of nuts after harvest.
When cut open, rotten chestnuts usually show a tan-brown discolouration of part or all of the nut kernel. Rots can range in colour from chalky-white to dark-brown.
Early stages of rots are even less obvious. Small reddish-brown spots may be visible on the pellicle (the papery layer beneath the shell); these may or may not extend onto the kernel. Fine-brown specks or lines may appear inside healthy-looking cream-coloured kernel tissue.
Figure 1. Rotted nuts (cut in half) compared to the healthy nut in top right hand corner.
Biology
Survival
The biology of P. castanea is not yet fully understood. P. castanea is mainly evident post-harvest, causing brown rots in mature nuts. However, the fungus has been found throughout the year in leaves, wood and flowers which display no disease symptoms. The fungus can grow inside the tree without causing any symptoms (it is endophytic) until rot develops in maturing nuts. Rot is sometimes found while nuts are still on the tree, but more commonly is found on nuts after they have fallen. P. castanea has been found after harvest in fallen leaves, twigs and burrs, but the significance of this is unknown.
Dispersal
Dispersal mechanisms are not fully understood, but the fungus appears to spread within the tree by growing through internal tissues.
The disease can develop in seedlings grown from infected nuts and can probably also spread in nursery stock which carry the fungus. Infection has been found in nuts from trees growing throughout the main chestnut production areas.
Phomopsis spores can be water splashed, so dispersal within the canopy of the tree could occur during rainy periods when rotted nuts are still on the tree. Spread may also occur within post-harvest water handling and via equipment, although the importance of this has not been demonstrated.
Environmental conditions
Cool-wet summer and autumn weather favours P. castanea, and hot-dry conditions during the growing season restricts disease development. As for most diseases of stored produce, Phomopsis rot is restricted by storage at low temperatures (around 0°C) and by maintaining the cold chain during distribution and marketing.
Host range
Only chestnut trees and nuts appear to be affected.
Control
Recent research has investigated both field and post-harvest treatments. Foliar sprays of certain chemicals have given significant but incomplete control of Phomopsis nut rot. Contact the Chestnut Growers of Australia Ltd for details of approved treatments.
Similarly, post-harvest treatments including hot water dips, modified atmosphere environments prior to storage, and packaging materials can reduce the incidence of Phomopsis nut rot during storage. Different cultivars were assessed for comparative rot incidence, and the cultivars Purton’s Pride, Red Spanish and Lucente showed less rot than cultivars Marone, Sword and Buffalo Queen. Although current control strategies for Phomopsis nut rot of chestnuts are limited, the following practices can reduce the incidence of rots in the marketplace:
- Harvest frequently (daily if possible) to minimise the delay between nut fall and storage/marketing.
- Store nuts in cold storage (around 0°C or -1°C) immediately after harvest.
- Check the quality of nuts before marketing. Take a random sample of the consignment, cut nuts in half and assess them for rot development.
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Acknowledgments
This work was part of a project supported by the Horticultural Research and Development Corporation and Chestnut Growers of Australia Ltd.
Correct diagnosis is essential for effective pest and disease control. A commercial diagnostic service is available at the DPI PIRVic Knoxfield Centre. For further information phone Crop Health Services on (03) 9210-9222 or fax (03) 9800 3521.
The previous version of this note was published in December 1999.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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