Inheritance of resistance to powdery mildew (Oidium lycopersicum) and its linkage to an SSR marker in tomato hybrid DRW4409

Powdery mildew is a serious disease for greenhouse and field tomatoes in North America. The main objective of this experiment was to study the genetic inheritance of resistance to powdery mildew in tomato hybrid DRW4409 and to identify simple sequence repeat (SSR) markers linked to the resistance gene. Analysis of a genetic population derived from DRW4409 showed that resistance to powdery mildew is controlled by a single dominant gene. Screening 158 SSR loci found that the SSR marker, LEat014, is linked to this gene at a map distance of 8.0 cM. To the best of our knowledge, this is the first report of an SSR linked to the resistance gene in DRW4409. Because of its co-dominant nature, this SSR should be useful to breeders in screening tomato plants for resistance to powdery mildew when DRW4409 is used as the resistance gene source.

Host Range of Oidium lycopersicum on Selected Solanaceous Species in Connecticut

Powdery mildew of tomato in Connecticut was identified as Oidium lycopersicum based on anamorph characteristics. On tomato, conidia were most often formed singly or in pairs on unbranched conidiophores. Mycelium was hyaline and measured 5 to 7 μm. Cleistothecia were not found. This fungus caused disease on a number of tomato cultivars ranging in size from seedlings to mature plants under greenhouse and field conditions. Eastern black nightshade, eggplant, tobacco, and potato were infected in the greenhouse. Reciprocal inoculation experiments between tomato and nightshade and eggplant and tobacco demonstrated cross-infectivity, but the size and number of conidia per conidiophore varied between hosts. Conidia from tobacco, eggplant, and nightshade averaged 37.8 by 19.1 μm (SE = 0.5 and 0.2, respectively) with a 95% confidence interval (CI) of 35.5 to 40.0 by 18.1 to 20.3 μm (n = 300). Conidia from tomato were smaller, averaging 33.6 by 18.2 μm (SE = 0.3 and 0.2, respectively) with a 95% CI of 32.7 to 34.4 by 17.5 to 18.9 μm (n = 300). Conidia from potato averaged 32.4 by 16.6 μm (SE = 0.4 and 0.2, respectively) with a 95% CI of 31.5 to 33.3 by 16.3 to 17.0 μm (n = 100). There were significant differences in percent leaf infection and powdery mildew disease ratings between Lycopersicon esculentum cultivars and Lycopersicon breeding lines under both greenhouse and field conditions. Most commercial tomato cultivars were severely affected by powdery mildew. Cultivars such as Rutgers were not as extensively colonized by O. lycopersicum, compared to cultivars such as Celebrity and Nema 1200, but the severity of powdery mildew was still quite high. L. hirsutum and the tomato breeding line Hirol 3-21 were the only plants tested that were resistant to O. lycopersicum.