SCREENING TECHNIQUES AND SOURCES OF RESISTANCE TO PARASITIC ANGIOSPERMS

Parasitic angiosperms cause great losses in many important crops under different climatic conditions and soil types. The most widespread and important parasitic angiosperms belong to the genera Orobanche, Strig a, and Cuscuta. The most important economical hosts belong to the Poac eae, Asteraceae, Solanaceae, Cucurbitaceae, and Fabaceae. Although som e resistant cultivars have been identified in several crops, great gap s exist in our knowledge of the parasites and the genetic basis of the resistance, as well as the availability of in vitro screening techniq ues. Screening techniques are based on reactions of the host root or f oliage. In vitro or greenhouse screening methods based on the reaction of root and/or foliar tissues are usually superior to field screening s and can be used with many species. To utilize them in plant breeding , it is necessary to demonstrate a strong correlation between in vitro and field data. The correlation should be calculated for every enviro nment in which selection is practiced. Using biochemical analysis as a screening technique has had limited success. The reason seems to be t he complex host-parasite interactions which lead to germination, rhizo tropism, infection, and growth of the parasite. Germination results fr om chemicals produced by the host. Resistance is only available in a s mall group of crops. Resistance has been found in cultivated, primitiv e and wild forms, depending on the specific host-parasite system. An a dditional problem is the existence of pathotypes in the parasites. Inh eritance of host resistance is usually polygenic and its transfer is s low and tedious. Molecular techniques have yet to be used to locate re sistance to parasitic angiosperms. While intensifying the search for g enes that control resistance to specific parasitic angiosperms, the be st strategy to screen for resistance is to improve the already existin g in vitro or greenhouse screening techniques.