Col Rot and Root Rot of Apples in Nelson and Marlborough

New Zealand Journal of Agriculture, Volume 96, Issue 1, 15 January 1958, Page 45

Col Rot and Root Rot of Apples in Nelson and Marlborough

By

C. E. WOODHEAD,

Pip Fruit Specialist, Department of Agriculture, Palmerston North

MANY Cox's Orange Pippin trees die each year in New Zealand apple orchards from collar rot, caused by the fungus Phytophthora cactorum, first identified in New Zealand in 1950 by Smith (I). Though diseased trees may be found in most if not all apple-growing districts, loss of trees is especially heavy in Nelson Province. These continuing heavy losses of Cox s Orange trees, coupled with the almost complete failure of remedial measures and the uncertainty of success when replanting in infected ground, are a serious threat to production of this important export variety. To secure a more accurate estimate of the extent of collar rot and to obtain information on predisposing factors and how the disease may be controlled or its incidence reduced, the survey described in this article was planned by the author and carried out by field officers of the Horticulture Division of the Department of Agriculture in Nelson and Marlborough during the spring and early summer of 1955-56. . .

AS the purpose of the article is to give the results of the survey, only brief reference will be made here to symptoms shown by infected trees. Attention is usually attracted to an infected tree first by failure of a portion of the tree to break into growth in spring. Alternatively, bud movement of one or more leaders may be late, subsequent growth is weak, and the foliage sparse and yellow (see •illustration on page 47). ■

A superficial examination of the trunk may show nothing unusual, but removal of the outer layer of bark just above the bud union 'will reveal an underlying area of rotted, infected tissue, black or dark brown, merging into lighter brown toward the margin of the canker, where it joins healthy, white tissue. , Illustrated at right is a typical canker, where infection has spread into the branches. As the disease progresses the trunk is completely girdled and death of the tree follows. Though in Cox’s Orange trees collar rot of the trunk above the bud union is the usual symptom of infection by Phytophthora, earlier observations on other varieties have indicated that this fungus attacks chiefly the roots and trunk below the bud union, that is, the portion of the tree provided by the rootstock.

SCOPE OF SURVEY As there was not time for a survey of all Cox’s Orange orchards, a sample selection was made of orchards at Nelson Mapua, and Motueka, and at Blenheim in Marlborough. This selection was . designed to give adequate representation of orchards on alluvial soils and on the: Mapua clay typical of the Moutere Hills, on which most of the orchards in the three Nelson districts have been planted. As the former are flat or slightly undulating and the latter gently to steeply rolling, this selection also provided for an

estimate of the effect of ground slope on the incidence of collar rot. Also included were a number of orchards in permanent grass, a recent development in orchard-soil management. Identification of collar rot (infection above the bud union) in the orchard presented few difficulties. Time available did not allow examination of the roots of all trees showing symptoms of root rot (infection below the bud union). However, Plant Diseases Division of the Department of Scientific and Industrial Research made many positive identifications of Phytophthora in root specimens forwarded to it, indicating that this

fungus was responsible for much of the root rot recorded. Trees were grouped according to age and whether the variety was single-worked or topworked. The term “single-worked” is used in this article to denote trees in which the variety had been budded or grafted directly on to the rootstock, in contrast to topworked trees in which a previous variety had been reworked to the present variety. In the topworked tree the trunk and usually portions of the main arms of the former variety remain to form an intermediate stem between the rootstock and the top variety. As will be shown later, this intermediate stem proved to be an important factor in the incidence of collar rot. RESULTS OF SURVEY Collar Rot in Cox's Orange The seriousness of collar rot in many orchards is shown by the fact that in 15 of the 69 h orchards more than 15 per cent, of the trees were infected,

and some orchards had over 40 per cent, infection. Incidence in Relation to Age and Type of Tree Single-worked trees: Of a total of about 18,600 single-worked trees, 10.4 per cent, were infected with collar rot. A significant feature was the relation of tree age to incidence, as was shown by the progressive increase of incidence from 0.2 per cent, in trees up to 5 years of age to 17.7 per cent, in trees over 40 years. Mapua, with 22.6 per cent., had a much higher percentage of collar rot in single-worked trees than other districts, and more infection was recorded in Blenheim orchards than in Nelson and Motueka orchards. Topworked trees: The low incidence of collar rot, 2.6 per cent., in the four districts’ total of 6095 topworked trees was only a . quarter of the infection recorded for single-worked, trees and confirmed earlier reports of field officers that Cox’s Orange topworked on other varieties was relatively free from collar rot. Results also indicate that, unlike single-worked trees, topworked trees up to 40 years of age have an insignificant incidence of collar rot. Of 191 single-worked Cox’s Orange trees on Northern Spy stock in this orchard, 75 have been replaced owing, to collar rot. Only one tree has been lost of the 58 Cox’s Orange topworked on Statesman. Both blocks of trees were planted in alluvial soil and are growing under similar conditions. The incidence of root rot in singleworked Cox’s Orange trees was very

low compared with that of collar rot. In this connection it was noteworthy that in Cox’s Orange trunks spread of Phytophthora infection downward past the bud union, that is, into the rootstock, was rarely observed. Collar Rot in Other Varieties It is clear from this survey that topworking Cox’s Orange on other varieties gives a very large measure of protection against collar rot. However, if full advantage is to be taken of this knowledge in future planting it is also necessary to know what varieties are likely to be satisfactory intermediates for both resistance to collar rot and compatibility with Cox’s Orange. The survey showed that collar rot was insignificant in single-worked trees of all other varieties with the exception of Gravenstein, which had 2 per cent, of trees infected. In topworked Cox’s Orange trees Ribston Pippin as the intermediate

variety was more severely affected by collar rot than was the single-worked Cox’s Orange. As the Cox’s Orange was raised from a seed of Ribston Pippin, its susceptibility to collar rot would appear to have been inherited from the Ribston parent.

There was a fairly high incidence, 5.1 per cent., in trees on Stunner intermediate, confirming the results obtained by Smith (2) in inoculation tests on this variety. Nevertheless, the fact that Sturmer is much more resistant than Cox’s Orange to collar rot is well shown in the illustration on page 49. Here healthy Golden Delicious topworked on Sturmer are seen in a Blenheim orchard growing close to a dying Golden Delicious tree topworked on Cox’s Orange. The illustration on page 51 is a near view of the latter tree, showing infection of the Cox’s Orange intermediate by collar rot. Of the 66 Golden Delicious trees topworked on Cox’s Orange in this orchard, 35 were infected with collar rot, in marked contrast to the 70 trees of the same variety topworked on Sturmer, all of which were healthy. This example clearly illustrates the risk of reworking Cox’s Orange trees to another variety where collar rot is prevalent. Though no infection was found in trees topworked on six of the intermediate varieties observed, in each case only one block was available for observation. Consequently, less reliance can be placed on the data recorded for these varieties than for varieties where observations were

made on several blocks and a larger number of trees.

In the latter category, trees on Dunn’s Favourite intermediate (11 blocks), and Statesman intermediate (4 blocks), with respectively 2.3 and. 1.7 . per cent, infection, were highly resistant when compared with singleworked Cox’s Orange trees of the same age group with 12.1 per cent, infection. It appears that either of these 'two varieties would provide a resistant intermediate for future propagation of Cox’s Orange trees. Observation of the growth of Cox’s Orange topworked on the two varieties indicated that they are also . highly compatible. Collar Rot and Soil Type Records were made of collar rot in Cox’s Orange single-worked trees on alluvial and clay soils. The alluvial soils included several types from different parent material; the clay soils, on the other hand, were of one type —the Mapua clay typical of the Moutere Hills.

Results showed that in the Nelson and Motueka districts collar rot was more prevalent on clay soils, but that there was more collar rot on the alluvial soils of Blenheim than in Nelson and Motueka orchards on clay. The highest incidence of collar rot, 25.9 per cent., was recorded in orchards on clay soils at Mapua. When compared with 5.2 and 6.9 per cent, respectively on the same soil at Nelson and Motueka this is difficult to explain, but the fact that the. majority of trees at Mapua are more than 40 years of age may be partly responsible. Collar Rot in Grassed Orchards Incidence of collar rot in Cox’s Orange was observed in seven , grassed orchards. As only one of these orchards had been grassed as long as 3 years and three were grassed in the year preceding the survey, no final conclusions could be drawn, but the evidence . appeared to be against any effect of grassing.

Other Factors A survey of other environment and management factors showed clearly that poor drainage is the dominant factor in the incidence of collar rot in single-worked Cox’s Orange trees, and this conclusion is supported by the fact that of four single-worked Cox’s Orange orchards with a history of no collar rot infection two were planted on a well-drained sandy loam and a third on dry, stony soil. Degree of ground slope also appears to be related to incidence of collar rot, as orchards on gently sloping ground had higher infection than those on steep slopes. As slope and its effect on drainage are significant factors in reducing incidence, one would expect to find a high proportion of the severely infected orchards on flat land. The survey did not show this. On the contrary, field officers when noting special features asso-

dated with the particular location of trees infected with collar rot frequently recorded pockets of infected trees in wet areas at the foot of slopes, in hollows on slopes, and in gullies between slopes. Therefore, wet trouble spots may occur with greater frequency in rolling country than on flat land.

As previously noted, soil type appears to be linked with collar rot incidence, infection of orchards on clay soil being in general, more severe. As with slope of ground, however, the connection between soil type and drainage cannot be disregarded. Other factors appear to be of little or no significance.

Under conditions of defective drainage risk of infection must obviously be increased where the union of rootstock and Cox’s Orange scion is at ground level or buried in the soil. This is clear from a comparison of collar rot infection in single-worked and topworked trees. During the survey it was observed that, with a few exceptions, the initial infection by Phytophthora had occurred in the Cox’s Orange wood at the bud union, which was in most instances at ground level or underground. In only a comparatively few trees was infection noted which had started at a point high up on the trunk or crotch, the maximum height recorded being 30in.

Root Rot in Other Varieties

The survey showed that in trees of the other main varieties susceptibility to root rot increases with age, little or no infection occurring in trees up to 10 years, but infection increasing thereafter. in a more or less regular pattern to the maximum recorded in trees more than 40 years of age. Delicious (see illustration on page 52), with 7.9 per cent, infected trees in this age group, was the most susceptible variety, closely followed by Gravenstein and Jonathan.

Root rot recorded for the four districts in older trees of the important varieties Delicious, Jonathan, and Sturmer showed a high incidence in all three varieties at Mapua. Infection was also severe in Nelson orchards. At Blenheim a high incidence was recorded in the variety Delicious, indicating that root rot is also serious in orchards on well-drained sandy loams. Infection was relatively slight in the Motueka district.

Root rot occurred in Delicious trees to a similar extent on both soil types, but was more prevalent in Jonathan and Stunner on clay soil.

The survey yielded little information on susceptibility to infection of different rootstocks, including East Mailing types, though root rot was recorded separately for trees on different stocks. Trees more than 10 years

of age were almost invariably on Northern Spy stock and, as already indicated, root rot in trees under this age was too insignificant for a correlation with rootstock type to be attempted. CONCLUSIONS FROM SURVEY Collar Rot The survey has shown that collar rot is a serious disease of singleworked Cox’s Orange Pippin trees in Nelson and Marlborough orchards, incidence being especially high in the Mapua district. The disease is of little significance in other varieties with the exception of Gravenstein. Though young trees are not immune, incidence of the disease in trees up to 20 years of age is low, but under conditions favourable to collar rot the

disease may become serious at any time after this age. It has been shown conclusively that topworking Cox’s Orange on other varieties reduces the risk of infection by at least 75 per cent, and that collar rot is insignificant in topworked trees up to 40 years of age. Defective drainage appears to be the dominant factor in increasing the disease. Though orchards showing high infection were most frequently located on clay soil and on gentle rather than steep slopes, probably these factors are important only to the extent that - drainage is detrimentally affected. The evidence from grassed orchards, though inconclusive, does not suggest that grassing is an effective remedy. There is also no clear indication that collar rot is increased where pre-

blossom copper sprays (bordeaux mixture) are not included in the spray programme. Root Rot The survey has. also revealed a high incidence of root rot in trees more than 40 years old of the varieties Delicious, Jonathan, and Sturmer, other varieties being affected in varying degree. It is also clear that the disease is prevalent in orchards on

both light and heavy soils and ‘ must obviously play an important part in limiting the . ; life of apple trees in commercial orchards. . v ■ RECOMMENDATIONS Until fully effective control of these diseases is possible, whether, by chemical treatment of the soil or by use of Phytophthora-immune rootstocks, the following recommendations should assist in reducing infection.

Treatment of Existing Orchards

To reduce the incidence of collar rot in Cox’s Orange trees, attention should be given to the following:—

Drainage, especially of clay land at the bottom of slopes and in gullies.

Promotion of drier conditions around trees by removal of grass and weeds in autumn, thus exposing the tree trunks to sun and air during winter and spring.

Preventing contact of the Cox’s Orange wood with the soil by ensuring that the bud unions of trees do not become buried. When clearing bud unions on shallow clay soil saucer-like depressions around the tree trunks, in which rain-water will accumulate, should be avoided.

Drainage is also important in the prevention of root rot and is at present the only treatment that can be suggested.

Future Orchards To give protection against collar rot future orchards of Cox’s Orange should he planted with trees of a type that will ensure that no Cox’s Orange wood is closer to the ground than about 36in. The raising of such trees in commercial nurseries would be difficult and costly. However, fruit growers have two alternatives. The first is to plant trees of a comparatively resistant variety Dunn’s Favourite or Statesman is especially recommended a year or two later rework the branched trees to Cox’s Orange at the height recommended.

The second alternative is to plant unworked rootstocks and when they are suitably branched graft them with Cox’s Orange. This method should obviously be used if stocks immune to Phytophthora infection become available.

Under present conditions, however, the first alternative is likely to save time, as rootstocks just off the stool are usually too sparsely rooted and slender to be headed back for branching until a year after planting.

On clay land the steeper slopes should be selected for the planting of Cox’s Orange, as they are likely to have better natural drainage. As a precaution against root rot the drainage of planting sites for other apple varieties should also receive attention.

Acknowledgments Acknowledgment is made to Dr. F. J. Newhook, Plant Diseases Division, Department of Scientific and Industrial Research, for identification of numerous disease specimens. References (1) Smith, H. C., 1950: Collar-rot of Apples and Gooseberries. “The Orchardist of New Zealand", vol. 23, No. 11, 1950-51. . (2) Smith, H. C., "Collar-rot and Crown-rot of Apple Trees”, “The Orchardist of New Zealand”, vol. 28, No. 10, 1955.