Raspberry Rootrots in Victoria		AG0163
W.S. Washington, Knoxfield Updated: January 2006
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A number of fungal genera have been shown to cause root rots of raspberry plants overseas including species of Phytophthora, Armillaria, Verticillium, Fusarium, Pythium and Rhizoctonia.

In Victoria, however, only two fungi have been shown to cause root rots in raspberry. White root rot, caused by a species of Vararia,was first recorded in Victoria in 1897. The other disease, Phytophthora root rot, is caused by several species of Phytophthora and has only been known on raspberry in Victoria since 1984. Both diseases cause a similar root rot and dieback, but white root rot differs in that the below-ground parts of infected plants are covered with a characteristic white fungal growth. Despite some similarities, these two diseases show many important differences in their behaviour. These are discussed below.

Host range and distribution

White root rot has been recorded from raspberry, loganberry, wild blackberry (Rubus fruticosus agg.) and plum, but serious losses have only been recorded on raspberry. Raspberry cultivars known to be susceptible include Lloyd George, Malling Promise, and Willamette, but no resistant cultivars have been identified.

The disease appears to be restricted mainly to the red mountain soils east of Melbourne, but has once been reported from Tasmania. It is not known outside Australia, although a white root rot of raspberry caused by different fungi has been reported from raspberry in New Zealand.

Phytophthora root rot of raspberry in Victoria is mainly caused by P. fragariae var rubi, and P. cryptogeathe first species being the most important. The relative susceptibility of raspberry cultivars to P. fragariae var rubi has not been determined. Tests by DPI with P. cryptogea found that all 12 cultivars of raspberry tested were infected by this pathogen, but a range of resistance to the disease was apparent. The cultivars Nootka, Chilcotin, Haida and Puyallup proved to be most resistant, while Glen Clova and Canby were the most susceptible.

Phytophthora root rot of raspberry has been recorded from plantings in southern and northern Victoria, and New South Wales, South Australia and Tasmania.

Symptoms

White root rot causes yellowing and premature autumn colouring of plant foliage, followed by rapid wilting and dieback. When young plants are affected, all the canes die simultaneously, but old bushes die gradually, a few canes at a time. The disease also causes a rotting of roots and canes below ground level. The most characteristic feature of this disease is that the affected parts below ground level are covered with a dense white to cream coloured mycelial mat. White fungal strands (rhizomorphs) develop on the surface of these parts.

Plants affected with Phytophthora root rot show similar symptoms to those caused by white root rot, but the white mycelial mat is absent. A characteristic grey or brown discolouration extending upwards from the roots may be seen in some plants when the outer bark is cut away.

Economic importance

White root rot is an important disease of raspberry, which in some cases has spread rapidly and practically destroyed entire plantations in three to four years. However, the disease more commonly spreads slowly, killing only a few bushes each year.

Phytophthora root rot is most important during wet seasons and in high rainfall, poorly drained plantings where infection is favoured. It has been found causing dieback in a number of raspberry plantings in Victoria, New South Wales, South Australia and Tasmania.

Disease cycle

Little is known about the disease cycle of white root rot as few studies have been made of this disease. The causal fungus is known to require a food base before being capable of infecting healthy plants, and can probably survive in soil for several years on infected raspberry roots or canes.

The disease is most severe when soil moisture is low and soil temperatures are high. Spread from diseased plants to adjacent healthy plants occurs mainly via root contact, but cultivation may also spread infected root and cane material. The disease can also be introduced to new sites through infected planting material.

Similarly, little is known about the disease cycle of Phytophthora root rot of raspberry, as few studies have been made of the disease. However, it is known to be favoured by wet soil conditions. Thus it is likely that wet seasonal conditions, heavy, poorly drained soils, or soils that are over irrigated should favour disease.

The fungus can persist in soils for several years, and can spread in irrigation or drainage water, or through saturated soil. This disease can be introduced to new sites through infected planting material and also in contaminated soil.

Prevention measures

Root diseases are particularly difficult to control, and every effort should be made to prevent them becoming established. The following factors are most important:

Choose sites that are suitable for raspberry production. Pay particular attention to soil structure, ensuring that the soil is deep and well drained and that adequate irrigation water is available. Where such soils are not available, considerable effort will be needed to improve the soil to a suitable condition. Ensure that planting material is of the best quality available and where possible, free from disease. DPI operates a certification scheme for raspberries, and aims to meet these standards. Treat soils that have recently grown horticultural crops such as raspberries, tomatoes and potatoes with a suitable soil fumigant before planting raspberries.

Control

The cause of dieback symptoms must first be established. Possible causes include drought, hot dry winds, waterlogging, weed competition, inadequate nutrition, foliar diseases, insect pests, nematodes and root rots. White root rot has quite distinctive symptoms (see above), but the presence of Phytophthora root rot or other diseases such as Verticillium wilt (common in raspberries in North America) is best confirmed by laboratory testing. Contact your local DPI office to arrange these tests.

Specific control measures for white root rot:

• Dig out and destroy infected plants and those immediately adjacent to these plants (to prevent spread of the disease)
• Fumigate replant sites where white root rot is present
• Provide adequate irrigation to prevent plants being water stressed.

• Avoid over irrigationm and improce soil structure and drainage e.g. plant into raised beds
• Plant more resistant cultivars.