Evaluating alternative nursery practices to reduce graft union failure.

Graft failure can be caused by factors such as the poor formation of the graft union (due to problems with anatomical mismatching, poor grafting technique, adverse weather conditions, and poor hygiene), mechanical damage, and graft incompatibility.

Our objectives to address these issues are a) to evaluate the effect of the length of the fresh cut of rootstocks pre-grafting on graft union failure, b) to evaluate the effect of biological and chemical fungicides on dormant commercial propagation materials against saprophytes and fungal pathogens of grapevine trunk diseases.

Evaluation of the length of fresh cut of rootstocks pre-grafting on graft union failure.

On a grapevine nursery, dormant rootstock material was cut at three different lengths (¼ in, 1 in, and 2 in) and further treated with biological and synthetic fungicides by soaking the cuttings in the fungicidal solution for 10 minutes. Treated cuttings were grafted by the nursery standard protocol and transported to the laboratory for callus evaluation. Since grafting involved waxing of the graft union, it was not possible to evaluate the callusing of this tissue. However, the basal end of rootstocks were evaluated by level of circumferential coverage of the rootstock apex by callus on a visual scale (1=0% callus coverage, 2=50% callus coverage, and 3=100% callus coverage). Data was analyzed by comparing the effect of the fresh cut length on the controls alone (Fig. 1) and in combination with the fungicide treatments (Table 1) using one-way and two-way analyses of variance (ANOVA). On the first one, no significant differences were observed in callus formation between the three cut lengths (Fig. 1).

When the two factors were analyzed, the interaction between fungicide treatment and fresh cut length was significative, meaning that the effect of the fungicides depends on the length of the fresh cut. Therefore, the effect of the fungicide treatments was compared within each level of fresh cut (Table 1). The callus formation reached 90%, 100% and 86% in average in the controls previously cut at ¼, 1, and 2 inch, respectively, and no treatment improved it significantly. Differences were detected between certain treatments among the different levels of fresh cut. Under ¼-inch cuts, Topsin M and HP Fitor+Phosful yielded significantly higher callus formation than VitiSeal, OxiDate (1%) and Vintec. With 1-inch cuts, Vintec significantly increased the callusing than VitiSeal, P. chlororaphis, OxiDate (0.39%) and HP Fitor+Phosful. And with 2-inch cuts, HP Fitor+Phosful and OxiDate (0.39%) showed significantly higher callusing than B. velezensis, VitiSeal, OxiDate (1%) and Vintec.