Conditions That Increase Latent Infection of Grafted or Non-grafted Tomatoes with Pseudomonas solanacearum

Abstract

Seedling roots of the susceptible tomato cultivar Ponderosa and the resistant rootstock cultivars LS-89 and BF-Okitsu 101, Ponderosa or LS-89 scions grafted on Ponderosa or LS-89 rootstocks were inoculated with Pseudomonas solanacearum. At 14 days after inoculation, seedlings were indexed for disease severity and harvested to recover the bacterial pathogen from stem tissues. Many of the resistant and susceptible seedlings did not show wilting symptoms but were latently infected with P. solanacearum. As plant age at the time of inoculation increased, and inoculum concentration or incubation temperature decreased, the frequency of latent infection and wilting decreased. In LS-89, plants grown at 25/20°C (day/night) showed almost no symptoms but had latent infection at 20-75%. Against five strains of P. solanacearum, the percentage of wilting and latent infection of seedlings was not different in Ponderosa or LS-89, but it was significantly different in BF-Okitsu 101. Both Ponderosa and LS-89 scions grafted on Ponderosa had nearly all wilted. However, Ponderosa scions gratfed on LS-89 wilted at nearly 60%, whereas LS-89 scions grafted on LS-89 showed no wilting. Sixty-nine percent of these symptomless LS-89 scions and 35% of the symptomless Ponderosa scions did have latent infections. These results indicated that bacterial wilt of susceptible scions grafted on resistant rootstocks was due to a movement of the pathogen from latently infected rootstocks into the scions.