Cause Hazelnuts will not tolerate saturated soil conditions for extended periods of time during the winter. Trees grow poorly under these wet conditions for years and eventually may die.
The OSU Plant Clinic has detected the fungus-like microorganism Phytophthora from the fine roots of young, dying trees. Although this is considered rare in hazelnuts, Phytophthora is a common root rot organism found on many other plants growing in saturated soil conditions. Phytophthora root rot has also been observed in BC on 'Jefferson' and 'Theta'.
Symptoms Trees growing in low areas of the orchard or swales are of low vigor and slow growing relative to other trees in the orchard. Trees may survive for many years in these areas but remain stunted and do not produce as many nuts. Some trees will continue to decline with a dieback of twigs and branches and eventually die.
Phytophthora infection can result in discolored roots and vascular cambium as it colonizes the roots and root crown. Fine roots are discolored and decayed with loss of the root cortex. Young trees may not survive the first year or two after planting and die. Mature trees may develop a dark wet stain on the lower trunk. These areas may weep a dark fluid in the spring. Diseased and discolored cambium will extend down to and below the ground rather than up and into the branches. General decline symptoms above ground of yellowing, branch dieback, and nutritional deficiency will occur from lack of functioning roots. Trees may show lack of vigor and suddenly crash during the summer when hot weather prevails.
Cultural control First determine the soil type and structure, as that will help guide the choice of corrective measures.
- Plant trees on well-drained soils.
- Tile low areas of the orchard for improved water drainage.
- Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
Chemical control Phytophthora root rot might be controlled using the following products that are effective on other crops. Only use when diagnosis is confirmed. These chemicals will not keep heavily infected trees from dying. Use in the spring when root growth is just starting.
- Agri-Fos at 2.5 quarts/A. Do not combine with a copper-spray program for control of other diseases. WA only. Group P7 fungicide. 4-hr reentry.
- Fosphite at 1 to 3 quarts/A. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Orondis at 4.8 to 9.6 fl oz/A as a drench, soil-directed spray or through irrigation water. Do not use within 30 days of harvest. Group 49 fungicide. 4-hr reentry.
- Ridomil Gold SL at 4 pints/A applied to the soil or through irrigation system before growth in spring. Do not use within 30 days of harvest. Will not save trees with extensive damage. Group 4 fungicide. 48-hr reentry.
Reference Guerrero, J.C., Perez, S.F., Ferrada, E.Q., Cona, L.Q., and Bensch, E.T. 2014. Phytopathogens of hazelnut (Corylus avellana L.) in Southern Chile. Acta horticulturae 1052:269-274.
O'Dell, T.E. 2019. Flowering and Yield of Eastern Filbert Blight-Resistant Hazelnut Cultivars in Southwest British Columbia. Canadian Journal of Plant Science 99:480-487.