Verticillium Wilt Management: Elucidating Mechanisms of Resistance and Integration of Sustainable Alternatives in Tomato Production Systems
Verticillium wilt (VW) is a serious disease in the Mountain production region of NC and surrounding States. No genetic resistance is available to the race 2 form of the pathogen (Verticillium dahliae) and this race is widely prevalent. This project hypothesized that vigorous rootstock combined with grafting protocols could reduce VW problems to compliment or replace current fumigation methods. The cultivar ‘Mountain Fresh’ was used as the scion in all experiments. In soils with high VW pressure in 2012 at an on-farm research site, rootstocks ‘Maxifort’ and ‘OH316’ reduced VW incidence compared to the non-grafted control. Several rootstocks had severe VW incidence and the non-grafted control was intermediate. Plants with severe VW incidence had low yields whereas the non-grafted control and vigorous rootstocks had superior yields. At a second site, low VW pressure occurred and major differences in plant growth parameters, yield and VW incidence were not observed. Research efforts were translated into extension programs and several highly successful grafting training workshops were offered across the State.
There were three objectives for this project. The first was to evaluate the utility of rootstocks as a viable mechanism for growers in managing Verticillium wilt in either fumigated or non-fumigated soils. The second was to advance the current knowledge on the mechanism of host resistance of the rootstocks through intensive field sampling, PCR analysis, and greenhouse assays. The third and final objective was to communicate our results to growers and other clientele with the recommendations of usable rootstocks for integrated and sustainable tomato production systems.
Five different rootstocks were grafted to one universal scion (Table 1) and were evaluated at two field sites in Western North Carolina on land with a history of severe Verticillium wilt (VW) pressure. One field site was located at the Mountain Horticulture Crop Research Station (MHCRS) in Mills River, NC while the other was located on a grower’s field in Haywood County (HC). The experimental design had 4 replications and was arranged as a split plot with 4 rootstocks and the non-grafted control randomly assigned as subplots within main plots of fumigation or no fumigation treatments. Parameters measured included weekly plant height, biweekly plant biomass, plant vigor, yield and Verticillium wilt incidence and severity.
Rootstock treatments did not affect plant height (Fig 1) or plant biomass (data not shown). There was no fumigation by rootstock treatment interactions in total marketable fruit at either site. ‘OH316‘ and a line from a Japanese company (V2), thought to have VW resistance, had the lowest yields at the on-farm research site; all other rootstocks and the control performed similar to each other (Figure 2A). At the MHCRS site, all rootstock treatments performed similarly except ‘Maxifort’ with low yields, apparently due to technical issues with the grafting process (Figure 3A). At both field sites, fumigation increased marketable yields (Figure 2B & 3B).
There was a significant fumigation by rootstock treatment interaction effect for plant vigor ratings at the on-farm site in the presence of high VW pressure (Table 2). Plant vigor increased significantly with the addition of fumigation for rootstocks ‘OH316’, ‘RST-DP106’, ‘V2’, and the non-grafted control ‘Mountain Fresh’. In contrast, there was no beneficial effect of fumigation for rootstocks ‘Maxifort’ and ‘OH313’. ‘Maxifort’ had high vigor ratings in both fumigated and non-fumigated plots, suggesting the vigor provided by this rootstock overcame the effect of no fumigation. The ‘OH313’ performed equally poor in both fumigated and non-fumigated plots (Table 5). Vigor effects due to treatment were not observed at the MHCRS site (data not shown).
Verticillium wilt incidence and severity was assessed at the on-farm site but pressure was low at the MCHRS site (data not shown). At the on-farm site, ‘OH316’ and ‘Maxifort’ had the lowest VW incidence compared to all other treatments (Figure 4). In contrast, ‘OH313’ and ‘V2’ had high levels of VW incidence and ‘RST-DP106’ and the non-grafted control ‘Mountain Fresh’ were intermediate. Verticillium dahliae was verified at the on-farm site through laboratory techniques. Colonies from soil and symptomatic plant tissue were grown on semi-selective media and were verified based on morphological characteristics: dark microsclerotia when examined with a dissecting microscope and presence of verticilliate arrangement of the branched conidiophores when examined with a compound microscope. Four representative colonies were selected for PCR amplification of a ?-tubulin gene fragment using V. dahliae specific primers and sequenced. BLASTED sequences confirmed DNA extraction from cultured plates to be a 100% match to V. dahliae.
Research experience on grafting was translated into extension programming. Multiple grafting workshops were conducted across the State to demonstrate the grafting process as well as educating about IPM strategies for disease control in tomato. Hands-on experience with the grafting process allowed growers to get firsthand experience grafting tomatoes using the tube grafting method.
Impacts and Contributions/Outcomes
With the completed field trials from the past two years, we were able to assess the utility of several rootstocks in the presence and absence of VW disease in field conditions. Growers would receive no yield or plant growth penalty with the use of grafted plants in the absence of disease (results from 2011). In the presence of VW pressure, vigorous rootstocks such as ‘Maxifort’ and ‘OH316’ can reduce disease incidence although dramatic yield benefits were not observed. Continued training of the grafting method and IPM strategies for disease control will help to expand grafting across the state and provide growers another IPM tool to manage Verticillium wilt.
Dept of Plant Pathology
Raleigh, NC 27695-7616