<p/>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 <em>Solanum pennellii, Lycopersicon esculentum</em> CStMW-18 and <em>L.</em><em>peruvianum</em> four strains are differentiated which correspond with Pelham's Strains 0, 1, 2 and 2 <sup><font size="-1">a</font></SUP>. 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 <sup><font size="-1">a</font></SUP>during isolation suggests that in practice it may not easily overcome the resistance of the gene Tm-2 <sup><font size="-1">a</font></SUP>. 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.
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