Phomopsis nut rot of chestnuts
Note Number: AG0513
Published: December 1999
Updated: August 2010
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. The disease is also widespread throughout chestnuts in New Zealand.
Chestnuts with advanced kernel rot may have a dark shell, are soft to touch and appear mouldy (Figure 1). Badly-rotted nut shells may be covered with small raised, cream-coloured structures which are the fruiting bodies of the causal fungus (Figure 2). 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 the entire nut kernel. Rots can range in colour from chalky-white to dark-brown (Figure 3). 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.
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 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.
Cool wet summer and autumn weather favours P. castanea, and hot-dry conditions during the growing season restrict 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.
Only chestnut trees and nuts appear to be affected.
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.
Contacts/services available from DEPI
For effective pest and disease control, correct diagnosis is essential. Phone Crop Health Services on (03) 9032 7515 or fax (03) 9032 7604.
This work was part of a project supported by the Horticultural Research and Development Corporation and Chestnut Growers of Australia Ltd.
This Agriculture Note was developed December 1999. It was most recently reviewed by W.S. Washington, Plant Standards in August 2010.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
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