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Various strains of TMV occur in susceptible crops of tomato. This variability is of importance to resistance breeding and to cross protection as alternative methods of control. Strains of TMV differ in the kind of symptoms expressed and in the extent of the host range. Symptom variation allows for the distinction of eight strains of which the tomato strain is the most important. On a host range including Solanum pennellii, Lycopersicon esculentum CStMW-18 and L.peruvianum four strains are differentiated which correspond with Pelham's Strains 0, 1, 2 and 2 a. Strains of TMV also differ as to longevity in vitro. The prevailing Strain 0 and Strain 1 an more persistent than Strain 2. The instability shown by Strain 2 aduring isolation suggests that in practice it may not easily overcome the resistance of the gene Tm-2 a. The symptomless nitrous acid mutant MII-16 which is discussed in relation to cross protection is also less persistent than its parent tomato strain. This necessitated a careful preparation of the inocula for commercial use.

Various aspects of soil suitability research in horticulture are explained by results from asparagus (a moderately intensive crop) and apple (more intensive) outdoors, and tomatoes in heated greenhouses (very intensive). The less intensive the crop, the clearer the limit the soil set in yields, because, for intensive crops, it was more economic for the grower to adapt to the soil by crop management and soil improvement.Yield, quality and the economics of asparagus were strongly influenced by the soil. Rooting depth was an important factor and depended on penetration resistance. In layers with a high resistance (30 kgf/cm 2), roots did not occur, as in C horizons of sandy soils. For the apple Jonathan on finetextured river clays, growers could allow for soil limitations by planting trees closer together. Tree growth was influenced more by a soil layer with a bad structure than was yield per m 2covered by the trees.In trials with tomatoes in heated greenhouses soil had little effect if management was optimum. More rapid plant growth and lower fruitset on the first trusses on sandy soil than on clay soils were explained by differences in available water. Water is stronger bound by clay soils than by sandy soils, but crop management could correct for differences between soils., Bodemkundig onderzoek Stiboka

In the Netherlands, the source of inoculum of the late blight fungus on tomatoes is the late blight fungus on potato crops. In regions of Europe mentioned, where tomatoes are grown in the open, P. infestans on tomatoes is the main source of inoculum. Especially in Bulgaria and Hungary, the effect of year-round cropping of tomatoes on prevalence and severity of the disease is obvious.In field and laboratory experiments, the pathogenicity to tomatoes of P. infestans originating from potatoes can be increased by serial passages through tomato foliage. In Europe, no other hosts than potatoes and tomatoes played a role in the tomato late blight epidemidogy. Phacelia tanacetifolia is added to the host list of the fungus. For the purpose of genetics classification, two laboratory methods, probit - log dosage analysis and components analysis, and one field method have been developed to assess the resistance of tomatoes against P.infestans.In particular the components infection ratio, lesion extension and sporulation intensity were studied. In the field method, the 'apparent infection rate' r was used. For breeding purposes, a simplified field method was indicated. A comparison of the three methods was made.The resistance of tomatoes increased considerably until at least 8 weeks after seeding. Partial resistance of W.Va 700 against the 1-t-pathotype of P.infestans is based on a single gene, here named Ph 2 .