Abstract
A description is provided for Peronospora rubi. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Rubus arcticus, R. caesius, R. canadensis, R. canescens, R. chamaemorus, R. cissiburiensis, R. cissoides, R. corylifolius egg. (R. caesius × tereticaulis, R. nemorosus), R. flagellaris, R. fruticosus egg. (R. bregutiensis, R. buschi, R. glandulosus, R. hirtus, R. menkei, R. seebergensis, R. tereticaulis, R. vulgaris), R. idaeus, R. laciniatus, R. leucodermis, R. × loganobaccus (orursinus × ideaus), R. occidentalis, R. parviflorus, R. plicatus, R. procerus, R. spectabilis, R. strigosus, R. sulcatus, R. tuberculatus, R. villosus, R. vitifolius, and certain hybrids, e.g. 'Tayberry' (blackberry cv. Aurora × tetraploid red raspberry), 'Tummelberry' (a 'Tayberry' interspecific cross) and 'Youngberry'. DISEASE: Downy mildew of cane fruits (Rubus spp.), especially blackberry (R. fruticosus agg.), boysenberry (a blackberry × red raspberry cross: the name R. × loganobaccus covers this plant) and raspberry (R. idaeus). The fungus, an obligately biotrophic plant pathogen, occurs on leaves in summer to autumn, producing small, conspicuous, irregularly shaped patches on upper leaf surfaces, starting near the petiole, then following leaf veins. Patches are initially yellow, becoming carmine-red, vinaceous or purple and are bordered by venation. The undersurface of the leaf shows only a pale area with a brownish edge, and brownish discoloration near and alongside veins. Sporophores are sometimes difficult to detect in the dense mat of leaf hairs, but are heaviest on lowest leaves, close to ground level, forming a buff-grey felt. In wild-growing European species of Rubus the fungus occurs exclusively on the leaves. In North America it attacks leaves of cultivated raspberry bushes, and in New Zealand the fruits, sepals and pedicels of boysenberry, causing the fruit to become dry and shrivelled (dryberry disease). Downy mildew has recently become a problem on certain berry cultivars in Eastern England (McKeown, 1988). GEOGRAPHICAL DISTRIBUTION: Africa: South Africa. Asia: USSR (Azerbaijan). Australasia & Oceania: New Zealand. Europe: Czechoslovakia, Denmark, Finland, France, Germany (GFR, GDR), Norway, Poland, Rumania, Sweden, Switzerland, United Kingdom, USSR (Latvia). North America: Canada (British Columbia), USA (IL, MD, OR, WA, WI). See CMI Distribution Maps of Plant Diseases 598. TRANSMISSION: Determined for boysenberry in New Zealand only (61, 4245), where it is a systemic disease confined to the outer cortex parenchyma, keeping pace with cell division at apical meristems. Systemic cane infection is often indicated by red streaking of stems and petioles linking successively diseased leaves on a shoot. Unfolding leaves are invaded during warm wet weather causing typical leaf symtoms. Stores produced on diseased shoots initiate secondary infections of flowers and developing berries. These berries then become an important source of inoculum for new cycles of the disease. They go largely unnoticed, since spores are partially hidden on the split berry surfaces or covered by the sepals. After harvest, infection of developing primocanes continues by internal mycelial growth and spore infection. Oospores form on root surfaces in dead cortex cells and leaves. Soilborne oospores may infect healthy plants established in former sites of infected root crowns.