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Spermogonia are small <1 mm diam) bright orange, slightly raised spots on the adaxial (upper) surface of leaves of primocanes (first-year canes) and mainly on the lower leaves of the lateral shoots of fruiting canes. Obvious yellow aecia appear later on the adaxial leaf surface as orange rings (0.5-2.0 mm diam) with the old spermogonia in the centre of the ring. A secondary aecial ring may develop around the first-formed ring. Yellow aecia sometimes occur on petioles, pedicels and sepals. Necrotic lesions may develop around the aecia leading to a 'shot-hole' effect as necrotic tissue falls out of the leaf.
Yellow uredinia appear on the abaxial (lower) leaf surface immediately beneath the old aecia on leaves of both primocanes and fruiting canes. Occasionally they appear on the internodes of primocanes and on petioles, sepals and mature fruit. Yellow uredinia appear throughout summer on new leaves of primocanes. Leaves affected by uredinia may show 'green islands' on the adaxial surface. In late summer and autumn black teliospores appear amongst the yellow uredinia. By late summer all the pustules are covered with black teliospores. Severely affected leaves may be shed.
Cane vigour control applied to reduce excessive numbers and height of primocanes in some cultivars reduces the risk of young canes being damaged by pickers and machines (Williamson et al., 1979; Lawson and Wiseman, 1981, 1983) reduces the severity of raspberry yellow rust because the primocanes are removed soon after they have been infected by the basidiospores that are released during a brief period in early spring (Anthony et al., 1987). The primocanes can be removed when they attain a height of 15 cm from soil level by cutting or with a contact herbicide. However, dinoseb-in-oil used initially for this method has been withdrawn on toxicological grounds and sodium monochloracetate is used in the UK as a cane vigour control agent. An extension of this approach to cane management is biennial cropping (Waister et al., 1977; Wright and Waister, 1979a, b) The biennial method involves the complete removal of all fruiting canes in one area with production of only vegetative primocanes that season; in another area of the field used for fruit production all primocanes are removed in successive flushes by means of an approved contact herbicide. Small-scale experiments indicate that rust can be reduced in comparison to levels recorded in conventionally grown raspberries and has the advantage that teliospores adhering to overwintering canes and old cane stubs would be greatly reduced in number (Anthony et al., 1987).
Culture of raspberries out-of-season under plastic and in glasshouses has increased recently in several European countries and this fungus has become more prevalent, probably because of the high humidity maintained in these structures. All measures to increase ventilation and reduce humidity will reduce the risk of infection.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Infection at the uredinial stage can cause some loss of vigour; when leaves are severely infected this rust causes premature defoliation and the disease may therefore increase the risk of severe winter injury in growing regions that experience hard winters.