Articles
PRACTICAL SITUATION AND FRESH HOPES FOR THE GENETIC CONTROL OF THE MAIN PATHOGENS OF THE MEDITERRANEAN FRESH MARKET TOMATO CROPS
Genetic control of resistance in the tomato relies on making use of the resistance of wild species closely related to the cultivated form.
Most of these resistance are monogenic and dominant.
This strategy is successful since many years in controlling the Tomato Mosaic Virus (= TMV), the fungi Verticillium dahliae, Fusarium oxysporum f. sp. lycopersici (2 pathotypes), Stemphylium spp. and Cladosporium fulvum (= Fulvia fulva) (several pathotypes) and the root knot nematodes Meloidogyne spp.. More recently, cultivars resistant to Fusarium oxysporum f. sp. radicis lycopersici (= FORL) or Pseudomonas syringae pv. tomato have also become available.
All modern tomato varieties for the fresh market cumulate from 3 to 7 dominant resistance genes.
Other pathogenes are more difficult to control genetically.
In some cases, this is due to the conditional expression of partial resistance, which is hard to select, as typified by resistance to Phytophthora infestans and the soil fungus Pyrenchaeta lycopersici, the latter resistance being moreover near-recessive.
Molecular markers would be highly useful for this type resistance.
There are strong hopes of genetic control suceeding against other pathogens of current interest.
On the powdery mildew front, breeders have started marketing F1 hybrids with a monogenic resistance to Leveillula taurica. Recently discovered resistances to Oïdium lycopersicum, are currently being incorporated into the cultivated tomato.
Several F1 hybrids resistant to the Tomato Yellow Leaf Curl Virus (= TYLCV) are now available and many breeding programmes have been initiated against this virus which has now reached the western Mediterranean Basin.
Breeders are trying to use the dominant newly described monogenic resistance to Tomato Spotted Wilt Virus (= TSWV). The first lines partially resistant to Liriomyza spp. are under observation.
The practical situation for exploiting various other widely studied resistances is hard to assess.
Information should be shared, primarily concerning resistances to Corynebacterium michiganense (Clavibacter michiganensis), Xanthomonas vesicatoria, whiteflies and aphids.