Why is it so difficult to identify tomatoes resistant to late blight?

Late blight (Phytophthora infestans) of tomatoes (Solanum lycopersicum) has been sporadically important throughout the world and has recently caused devastating epidemics on tomatoes in diverse locations worldwide. Most tomato cultivars are susceptible. Three R genes have been used commercially and provide some protection against certain pathogen populations. However, R gene resistance is effective only against certain pathogen populations, and can “break down” when the pathogen population changes - sometimes revealing that previously resistant cultivars have become highly susceptible. For resistance that is less dramatic than that provided by R genes, the technique used to screen plants for resistance can influence the results of inoculation studies using sporangia. Sporangia obtained from a pure culture on an agar medium can produce different results than sporangia prepared from infected plants. This is because sporangia of P. infestans that are produced in pure culture on an agar medium are dramatically less aggressive and less ecologically fit than are sporangia produced on infected leaflets. Two examples illustrate the situation. The first example concerns elicitins NDASH small proteins from P. infestans that induce a hypersensitive response in host plants and have been suggested to induce host resistance. Most genes that code for elicitins are upregulated in sporangia produced in pure culture on an agar medium, and elicitins may induce resistance to such sporangia. However, most elicitin genes are downregulated in sporangia from leaflet lesions. Thus, it seems unlikely that elicitins induce resistance in hosts under natural or agricultural conditions. The second example concerns RxLR effectors NDASH thought to be important to pathogenicity by P. infestans. The genome of P. infestans has more than 550 effectors. Sporangia from a pure culture on agar medium produce a very different suite of RxLR effectors than do sporangia from a leaflet lesion. Those that might be involved in early stages of pathogenicity may be downregulated in sporangia from a pure agar culture. Thus sporangia from a pure culture on an agar medium are in a very different physiological state than are sporangia from a leaflet lesion, and have different capabilities. Because every sporangium in an epidemic in the field comes from a leaflet, it seems prudent to use such “natural” leaflet sporangia in assays to assess host resistance.