Yellow Leaf Spot and Fruit Pimpling and Cracking of Nectarine and Peach

New Zealand Journal of Agriculture, Volume 99, Issue 5, 16 November 1959, Page 423

Yellow Leaf Spot and Fruit Pimpling and Cracking of Nectarine and Peach

By

W. S. KEMP,

Horticultural Instructor, Department of Agriculture, Alexandra

HORTICULTURAL RESEARCH

SOME varieties and blocks of nectarine trees in the Alexandra district have for many years produced malformed, spotty, and cracked fruit. Though damage is not of the same intensity every year, incidence appears to be increasing. Attempts at eliminating the damage, mainly along the lines of a possible trace element deficiency, have been unsuccessful. Recent trial work has shown promising results on both Hunt's Tawny and Goldmine nectarine and seems to point to the fruit injury being caused by eriophyid mite or capsid bug damage in the early stages of fruit development.

yHOUGH the trials are not yet conelusive, publication of results to date may enable growers with similar fruit conditions to undertake their own trials and thus give a wider field from which information could be obtained.

Though the preliminary work has been confined to Hunt’s Tawny and Goldmine, most other nectarine varieties are known to show similar symptoms in some orchards. Cardinal and John Rivers are two varieties sometimes showing severe symptoms.

Fruit symptoms vary from a reddish spotting or scabbiness to severe cracking and distortion, often with gum exudation (Fig. 1). There are also usually leaf symptoms, though these are more apparent in some varieties than others. They comprise a yellow

spotting of the leaf (Fig. 2), and often some distortion. In addition to nectarines many peach varieties show similar leaf symptoms, though no fruit symptoms occur. However, fruit symptoms (Fig. 3) of possible insect origin have been observed in association with leaf spotting and this point should be kept in mind when similar problems occur in the future.

After early work on the possibility of trace element deficiency, mainly boron, the possibility. of its being of virus origin was investigated, but these trials were negative. Consideration was given to insect attack and suggestions were made to growers to try nicotine sulphate. Last season, though no definite insect was suspected, a trial was laid down on one of the worst affected blocks of Hunt’s Tawny. Two newer insecticides were used, the organo phosphorus compound methyl - demeton and endrin, applied at the petal fall stage. Results on this trial were inconclusive, but though differences between treatments were not significant, there seemed to be some benefit from the spray treatments. The trial was repeated in the spring of 1958 with the same materials, methyldemeton at 1 pint in 100 gallons and endrin at 1 quart in 100 gallons, applied at pink and petal fall. There was a very definite response. Trees in both treatments were almost completely free from leaf and fruit symptoms, while adjacent untreated trees were severely affected, some to such an extent that no salable fruit was produced (Fig. 4). Leaves from affected trees sent to Plant Diseases Division, Department of Scientific and Industrial Research, were found to be infested with the peach silver mite (Vasates cornutus (Banks) of the family eriophyidae, which was considered by Plant Diseases Division to be responsible for the yellow spots on the leaves, though it was doubted if they were also responsible for the fruit symptoms. Thus the possibility of some other insect also being associated with this problem cannot yet be entirely eliminated. According to Plant Diseases Division the mites move from the leaves to leaf and fruit buds in early autumn and overwinter there. Thus there is a good argument for post-harvest and dormant sprays of lime sulphur or wettable sulphur, as sulphur is generally very toxic to eriophyids. The increasing incidence of leaf spotting in this district may be partly because sulphur sprays for the control of fungous diseases are no longer common practice with peach and nectarine growers. At the suggestion of Plant Diseases Division trial work is being extended this season to include some less expensive and less dangerous materials such as sulphurs and butylphenoxyisopropyl chloroethyl sulphite (“Aramite”). Proposed treatments are:

Lime sulphur in late autumn. Lime sulphur in late autumn and winter (dormant). Lime sulphur in winter (dormant). Wettable sulphur at pink and petal fall. “Aramite” at pink and petal fall. Methyldemeton at pink and petal fall. Endrin at pink and petal fall.

In addition specimens of any other insects which may be associated with this problem will be sought. Any nectarine and peach grower who has been troubled with similar fruit or leaf symptoms and contemplates trying any of the suggested treatments is asked to get in touch with the local Horticultural Instructor so that information on the effectiveness of the treatment can be recorded. Black and white photos by Department of Scientific and Industrial Research.