1 Dep. of Agricultural Sciences, University of Talca, Casilla 747, Talca, Chile; 2 Dep. of Biological Sciences, Wye College, University of London, Wye, Kent, TN25 5AH, UK

Background and objectives

Recent studies have indicated that apple trees can become infected with Nectria galligena (anamorph Cylindrocarpon heteronema ) in the nursery during propagation [1]. One possible source of the inoculum responsible for the development of cankers, particularly with highly susceptible scion cultivars, is the rootstock. A feature of such infection is that it can remain unexpressed for periods of more than a year. However, it is not clear why trees from a given batch of production from a nursery can develop cankers in the orchard in the first few years whilst others from the same batch remain symptomless. One possibility, identified by field observations is that stress caused by delayed planting and water loss predisposes the maiden trees to develop overt symptoms. A second possibility is that the difference in nutritional levels among apple orchards, mainly nitrogen, influences the time when the disease is manifested in the field. The objectives of the experiments described here was to determine the effect of delaying planting in trees from a commercial nursery and the effect of an application of excessive nitrogenous fertiliser on the incidence of canker in the subsequent season

Materials and methods

A completely randomised block design was used to test the effect of delayed planting dates on canker expression, with four replications of eight one-year old trees of Queen Cox on M.9 rootstock obtained from a single commercial nursery, and five treatments: 1) trees planted immediately after lifting (26/02/96); 2) trees planted after 2 weeks held in a cold room (2-3 C) at 92-95% relative air humidity (11/03/96); 3) after 4 weeks (25/03/96); 4) after 6 weeks (08/04/96); and 5) after 8 weeks (22/04/96). Trees were examined weekly for canker development. The second year after planting, the effect of nitrogen fertilisation on canker expression was tested within each plot. Trees were randomly divided in two groups receiving 0 and 40 g N/tree per year as ammonium sulphate applied broadcast with split dressings on 17 March and 27 May 1997. Additionally, in autumn 1997, the two groups were used to study the effect of nitrogen on the variability in pathogenicity of two isolates of N. galligena W68 and W74. Shoots were inoculated with a range of conidia concentrations of 0, 50, 100, 500, 1000 and 5000 conidia per leaf scar, removing approximately the fifth ant the seventh leaf and placing 20 ul of a conidia suspension on the scar with a micropipette.

Results and conclusions

When stored for 57 days, the tree weight loss reached 22.41% with the highest rate of loss occurring the first week of storage (7.2%). Moreover, there was a linear trend (p=0.036) in the decline of trunk diameter when maintaining them in a cold room after lifted from nursery. Tree losses associated with treatments were recorded. There was very strong evidence for a linear trend of increasing canker incidence with delay of plantation and cold storage after lifting from nursery (p=0.011). However, no further Nectria canker lesions were observed either in plots receiving N or in the untreated controls. N application did increase shoot length (p=0.05), but had no effect on trunk diameter. Artificial inoculation of leaf scars of shoots indicated that fewer conidia were required to induce lesions on trees receiving additional nitrogen than on controls. However the difference was small and not statistically significant. The water loss before planting would affect the normal tree establishment and development in the field. Ideally, trees should be lifted from nursery and immediately delivered to the planting site, but in practice plants may have to be cold stored or kept semiburied in the field. In this study we have shown that doing so, trees may become cankered. These results do not definitively link nitrogen to an active role in the expression of overt cankers.

References 1. Brown, AE, Muthumeenakshi, S, Swinburne, T, and Li, R. 1994. Detection of the source of infection of apple-trees by Cylindrocarpon heteronema using DNA polymorphisms. Plant Pathology 43, 338-343