ENVIRONMENTAL FACTORS IN THE EXPRESSION OF NECTRIA GALLIGEN IN APPLE TREES
M LOLAS1 and T SWINBURNE2
1Dep. of Agricultural Sciences, University of Talca, Casilla 747, Talca, Chile; 2Dep. of Biological Sciences, Wye College, University of London, Wye, Kent, TN25 5AH,
Background and objectives
Recent studies have indicated that apple trees can become infected with Nectria galligena (anamorph Cylindrocarpon heteronema ) in the nursery during propagation . One possible source of the inoculum responsible for the development of cankers, particularly with highly susceptible scion cultivars, is the rootstock (Swinburne and Langrell, in press). 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 (T. Swinburne, personal communication, 1995) is that stress caused by delayed planting and water loss predisposes the maiden trees to develop overt symptoms. The objective of the experiment described here was to determine the effect of delaying planting in trees from a commercial nursery on the incidence of canker in the subsequent season
Materials and methods
One year old trees of Queen Cox on M.9 rootstock with 3-4 feathers were obtained from a single commercial nursery. A total of 180 trees were used, each of which had been propagated under uniform conditions and were assumed to be a single 'batch'. On receipt the trees were randomly assigned to one of five groups of 36 trees. The first group was planted immediately, the remainder were held for various periods in a cold room (2-3°C) at 92-95% relative air humidity. At planting time each group of trees was further divided into sub-groups of eight and randomly assigned to a plot in each of four field blocks. Each plot (3x4 m) contained two rows of four trees (1 m between trees) and a straw mulched strip (0.5 m) with grass alleys (2 m) between rows. Spraying followed a normal orchard programme excluding pesticides which can suppress Nectria growth. Trees were well irrigated between middle of spring and middle of summer. The experimental design was a completely randomised block design, with four replications of eight trees and five treatments were applied: 1) trees planted immediately after lifting (26/02/96); 2) trees planted after 2 weeks (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 and bark samples were plated on 2% malt extract agar medium and DNA extracted and amplified.
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%). Water content decreased significantly (P<0.05) over time, varying among sections of the same tree, with roots and rootstocks below soil tending to be the highest. 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 more stressed treatments were registered. 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). 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.
1. Brown, AE, Muthumeenakshi, S, Swinburne, T, Li, R. 1994. Plant Pathology 43, 338-343.