Relative susceptibility of peach rootstocks to crown gall and Phytophthora root and crown rot in Chile

Crown gall (Agrobacterium tumefaciens) and Phytophthora root and crown rot (PRCR), caused by P. cryptogea, are two major limiting factors affecting peach (Prunus persica) production in Chile. At present, Nemaguard is the most important rootstock used in peach orchards, and although several clonal rootstocks are under development worldwide, there is little information regarding their susceptibility to crown gall and PRCR. Therefore this investigation was undertaken to study the susceptibility to A. tumefaciens and P. cryptogea of the clonally propagated rootstocks Cadman (R)-Avimag (P. persica x P. davidiana), Mr.S 2/5 (P. cerasifera x P. spinosa), Viking ([P. persica x P. davidiana] x [P. dulcis Jordanolo'x P. blireiana]), and Nemaguard (P. persica x P. davidiana), a seedling rootstock, which was included as a reference standard. Based on our results, Mr.S 2/5 was highly resistant to A. tumefaciens. Differences among Nemaguard, Viking and Cadman grown for 20 months in artifically-infested soil were not statistically significant, but only Cadman developed crown gall in soil with natural levels of inoculum. The extent of the canker lesions produced by P. cryptogea on excised twigs of each rootstock was greater on actively growing twigs than on dormant twigs. However, differences among rootstocks determined with the dormant twig assay correlated better with the results following trunk inoculations of 2-yr-old plants budded with 'Ruby Diamond' peach. For all rootstocks, PRCR severity increased considerably after repeated soil saturation episodes, and the loss of root mass increased in a linear manner as the saturation periods increased from 0 to 144 h. Our results showed that Mr.S 2/5 is relatively resistant to root rot caused by P. cryptogea, which is the most common Phytophthora species presently found on peach trees in Chile.