Effects of Different Tomato Mosaic Virus Resistance in Grafted Tomato Scions and Rootstocks

During the 2011-2012 production season, ‘heirloom’ tomato scions grafted onto Tomato yellow leaf curl virus (TYLCV)-resistant tomato rootstocks were observed to undergo a rapid and severe wilt, and necrosis.  No soilborne plant pathogens were detected, but several of the necrotic plants were infected with Tomato mosaic virus (ToMV).  During the 1970s, a rapid wilt resulting from differential virus resistance in tomato rootstocks and scions was reported in Japan.  This has not been of significant concern in the US where interest in grafting is relatively new.  In most cases, commercial tomato scions are resistant hybrids and have ToMV resistance.  The problem has arisen when ToMV-resistant rootstocks (having allele combinations of Tm-2/Tm-2 and Tm-2^a/Tm-2^a) are grafted with susceptible (Tm-1 or tm) scions.  In the case of the field trial using grafted ‘heirloom’ tomatoes where the rapid wilt occurred, scions were homozygous susceptible and the rootstocks were homozygous resistant with Tm-2/Tm-2 genotype.  Recent work has confirmed that, particularly with grafting of ‘heirloom’ tomato scions to commercially-available tomato rootstocks, it is important to select a rootstock that has the same resistance alleles as the scion, or has the Tm-1 gene, which appears to prevent the wilt regardless of the scion resistance status.  The wilt only occurs when plants become infected with the virus.  It has become standard practice in the Japanese market to provide ToMV compatibility information, but this has not been the case in the US, where the grafted tomato market is just beginning to grow.