S U M M A R YTeleutospores of Puccinia horiana Henn. germinate and discharge sporidia between 4 and 23 "C. At the optimum temperature of 17 "C sporidia discharge starts within 3 h. Maximum germination of the sporidia takes place within 2.5 h between o and 30 "C, there being no clear optimum. High humidity and a film of moisture appear to be necessary for germination of both teleutospores and sporidia. Sporidia can penetrate either leaf surface of chrysanthemum to cause infection between 4 and 24 "C and within the optimum temperature range, 17-24 "C, effectively penetrate within 2 h. The sporidia are very sensitive to desiccation at below 90% relative humidity.Methods are described, using leaf discs and whole plants, for screening chrysanthemum cultivars for susceptibility to white rust. Cultivars were placed in five classes ranging from susceptible to immune. Leaf discs of immune cultivars can be distinguished within 30 h by a brown discolouration at the point of inoculation. The early stages of development of the fungus in susceptible, resistant and immune hosts are described. The incubation period in susceptible plants is normally 7-10 days, teleutospores being formed a few days later. Leaves become less susceptible with age but the oldest leaves on 5-month-old plants could still be infected. T h e maximum survival time of teleutospores in the sori on detached leaves was 8 weeks but was considerably less under moist conditions or buried in soil. Low doses of a mancozeb with zineb fungicide controlled infection by preventing penetration rather than by inhibiting sporidial germination.

Global climate change, such as elevated atmospheric carbon dioxide (eCO2), may accelerate the breakdown of crop resistance to insect pests by compromising expression of resistance genes. This study investigated how eCO2 (700 μmol/mol) affected the susceptibility of red raspberry (Rubus idaeus) to the European large raspberry aphid (Amphorophora idaei) Börner (Homoptera: Aphididae), using a susceptible cultivar (Malling Jewel) and cultivars containing either the A1 (Glen Lyon) or A10 (Glen Rosa) resistance genes. Compared to plants grown at ambient CO2 (aCO2) (375 μmol/mol), growth rates were significantly increased (ranging from 42–300%) in all cultivars at eCO2. There was some evidence that plants containing the A1 gene were more susceptible to aphids at eCO2, with aphid populations doubling in size compared to the same plants grown at aCO2. Moreover, aphids grew 38% larger (1.36 mg compared with 0.98 mg) on plants with the A1 resistance gene at eCO2 compared with those at aCO2. Aphid performance on plants containing the A1 gene grown at eCO2 was therefore similar to that of aphids reared on entirely susceptible plants under either CO2 treatment. In contrast, aphids did not respond to eCO2 when reared on plants with the A10 resistance gene, suggesting that plants with this resistance gene remained resistant to aphids at eCO2.

A whole-plant method for screening chrysanthemum cultivars for resistance to white rust is described. Of 270 cultivars of the florists' chrysanthemum {Chrysanthemum sinense Sabine) tested, 93 showed some infection. According to the severity of infection these were rated as susceptible, moderately susceptible, resistant and very resistant. The remaining two thirds were rated as practically immune or immune.

. 1997. Genetics of resistance to alfalfa mosaic virus in red clover. Can. J. Plant Sci. 77: 601-605. Virus diseases are known to reduce the yield and persistence of stands of red clover (Trifolium pratense L.). Alfalfa mosaic virus (AMV) is one of the causes of mosaic in red clover and may also cause mottling and leaf distortion. The goals of this study were to identify genotypes resistant to AMV within commercial red clover cultivars and to determine the inheritance of this resistance. Cultivars and progenies were screened for resistance by means of mechanical inoculations. Plants showing no symptoms after three successive inoculations were assayed for the presence of the virus using ELISA (enzyme-linked immunosorbent assay). Resistant plants were detected at a frequency of just over 1% in the five cultivars screened. Crosses among and between resistant (R) and susceptible (S) plants were carried out over two successive cycles of selection. In cycle 1, R × R crosses produced a higher frequency of resistant plants (19.8%) in the progenies than R × S crosses (10.9%). S × S crosses produced no resistant progeny. For crosses of resistant plants from cycle 1 progeny, (R × R) × (R × R) produced 47.8%, (R × R) × (R × S) produced 31.4% and (R × S) × (R × S) produced 29.3% resistant cycle 2 progeny. Crosses of resistant and susceptible cycle 1 plants resulted in resistance frequencies in cycle 2 which were not significantly (P < 0.05) greater than cycle 1. When only resistant plants were selected for further crossing, the percentage of resistant plants increased from 1.3 to 47.8 over the two cycles of selection. It was concluded that resistance to AMV in red clover is under polygenic control and that it should be relatively simple to select a highly resistant population through recurrent selection.Key words: Red clover, Trifolium pratense L., virus disease, alfalfa mosaic virus, recurrent selection Martin, P. H., Coulman, B. E. et Peterson, J. F. 1997. Génétique de la résistance au virus de la mosaïque de la luzerne chez le trèfle rouge. Can. J. Plant Sci. 77: 601-605. Les maladies virales abaissent le rendement et la longévité des peuplements de trèfle rouge (Trifolium pratense L.). L'une des causes de la mosaïque chez le trèfle rouge, le virus de la mosaïque de la luzerne (AMV) provoque aussi le gaufrage et la déformation des feuilles. L'objet de nos recherches était d'identifier des génotypes résistants à AMV dans les cultivars de trèfle rouge et d'établir le mode de transmission héréditaire de cette résistance. Le tri était réalisé par inoculation artificielle des cultivars et des descendances. Les plantes ne manifestant pas de symptômes après 3 inoculations successives étaient testées pour la présence du virus par la méthode ELISA (dosage immunoenzymatique). La proportion de plantes résistantes décelée chez les 5 cultivars examinés était d'à peine un peu plus de 1 %. Des croisements entre et parmi plantes résist-antes (R) et plantes sensibles (S) étaient réalisés en deux cycles de sélection successifs. Dans le pre...

Virus diseases are known to reduce the yield and longevity of red clover (Trifolium pratense L.) stands. White clover mosaic virus (WCMV) is a commonly occurring virus in red and white clover (Trifolium repens L.). Tolerance to WCMV, as defined by a lack of symptom expression, has been detected in red clover. The objective of this study was to determine the inheritance of this tolerance. Progenies of red clover were screened for tolerance (T) to WCMV means of mechanical inoculations, and selection based on visual symptoms and ELISA (enzyme‐linked immnnosorbent assay). Average rates of tolerance to WCMV in progenies increased in two successive generations from 24.5% in T ✕ T to 34.2% in (T ✕ T) ✕ (T ✕ T), but in progenies of crosses involving susceptible (S) plants it remained almost constant: 8.4 and 9.7% in T ✕ S and (T ✕ S) ✕ (T ✕ S), respectively. Tolerance to WCMV behaved as it were controlled by polygenes. Percent tolerant progenies increased linearly through the two cycles of recurrent selection. It was concluded that a recurrent selection program would be effective in producing populations of red clover with high percentages of tolerant plants.