Phytophthora root rot of raspberry

''Phytophthora fragariae var. rubi'' is the principle cause of Phytophthora root rot of raspberry, specifically in red raspberry.

Symptoms
Outbreaks often start from small foci, increasing in size, especially down slopes. Symptoms usually appear on the upper parts of plants that come under stress in late spring or early summer. Some fruiting canes, i.e. canes in their second year, do not break bud; others break bud but their fruiting laterals wilt and dry out before or at fruiting. When the periderm round the bases of these canes is removed the wood underneath is usually discoloured reddish-brown or brownish-black. There is a dearth of young, first-year canes (primocanes); a very early and useful symptom is the absence in spring of a flush of primocanes in the alleyways between the rows of plants. Young canes wilt to give the appearance of a shepherd's crook. Their foliage becomes bronzed or reddish long before autumn (premature autumn colouring). Blackish-purple lesions can be found at the base of many young canes, best seen by removing the periderm, and these can extend for 20-30cm (8-12in) above soil level. The root systems of affected plants are badly rotted with few white feeder roots, and the thicker roots have internal discoloration often sharply demarcated from white unaffected regions of the root.

A number of other species of Phytophthora have been isolated from raspberries affected by root rot. P. cambivora and P. citricola may occasionally cause serious damage but most, such as P. cactorum and P. drechsleri, only cause significant damage in badly drained or waterlogged parts of a field.

Plants attacked by these other species are rarely as severely affected as those attacked by ''P. fragariae var. rubi'' and lack some of its characteristic symptoms, e.g. the blackish-purple lesions on young canes and large oospores restricted to the stele. Apart from some atypical isolates of P. cactorum, the other species have been isolated only at low frequencies from raspberries affected by root rot; they are not a major problem except in diagnosis.

Chemical
A number of fungicides are now approved for the control of raspberry root rot. Fosetyl-aluminium and related chemicals have not proved effective and only fungicides containing phenylamides have so far been approved, such as mixtures of metalaxyl and copper oxychloride, or metalaxyl and mancozeb. Applications are made in autumn and spring as band sprays directed at the soil at the base of the canes.

Cultural
Some control can also be obtained by good cultural practices, especially by improving drainage.

Resistance
In North America, a number of cultivars have useful levels of resistance: the red raspberries Newburgh, Meeker, Sumner and Chilliwack have some resistance while Latham and some black raspberry cultivars are highly resistant.

In Europe, where the disease has appeared only recently, resistance breeding is still at an early stage. Some autumn-fruiting cultivars such as Autumn Bliss appear to have useful levels of resistance but all summer-fruiting cultivars are susceptible, in most cases extremely so. Rubus spectabilis and R. parviflorus have high levels of resistance, and are being used in breeding programmes. The resistance of Autumn Bliss probably comes from R. spectabilis. Most hybrid berries such as tayberry which have blackberry in their parentage are highly resistant or immune, but loganberry is moderately susceptible. ''P. fragariae var. rubi'' has been isolated from tayberry but when reinoculated it caused moderate symptoms only after prolonged waterlogging.