Silverleaf (Chondrostereum purpureum) attacks a wide range of deciduous trees and shrubs. Apple trees may show the characteristic silvery symptoms but often the tree recovers. On stonefruit, however, the disease gets progressively worse until it eventually kills the tree.

In a recent survey by ENZA New Zealand (International) (ENZA Technical Update, July 1996), 78 % of the 831 respondents said they had noticed silverleaf on their orchard in the last three years. In general, growers were uncertain as to how silverleaf affected the fruit, although 40 % of the growers who responded to the survey considered silverleaf infection to affect the size and colour of the fruit. Thirty percent thought that silverleaf affected fruit set and fruit maturity and approximately 15 percent considered silverleaf to affect flesh firmness.

Now work by Adrian Spiers and his team at HortResearch, Palmerston North has shown that silverleaf does dramatically affect the yields and tree productivity of Fiesta apples and Golden Queen peaches.

Fiesta apples

During the 1995/96 growing season, all fruit from 5 pairs of healthy and moderately/ heavily silvered Fiesta apple trees were picked, measured and weighed. For this purpose, a "light" tree means silvering is not obvious, "moderate" refers to obvious silvering on most leaves, and "heavy" means the leaves are strongly silvered, shrunken and distorted. Table 1 shows that healthy Fiesta trees had higher yields and more fruit than from silvered trees. As there were fewer fruit on the infected trees, the average fruit weight was higher and the fruit were larger than apples from healthy trees.

Table 1. Influence of silverleaf infection on tree productivity

At harvest, 45 out of 60 (or 75%) of the sample fruit from silvered trees had watercore. The Fiesta picked from the healthy trees were less starchy, had lower soluble solids and tannins and were more acidic than apples picked from the silvery foliage, (click on Table 2 for maturity details at harvest). The high levels of water core in the fruit from the infected trees provided resistance to the pressure tester and may explain why the firmness levels are higher for these fruit.

Table 2. Fruit quality of Fiesta apples at harvest

When sliced and eaten after 10 weeks in coolstore, the fruit from the silvery trees tasted "floury". The reason for this was a change in cell structure ; cells in apples from silvered trees were collapsed and disorganised whereas those in fruit from healthy trees were hexagonal and well structured.

Figure 1: Cells of fruit from healthy Fiesta apple (left) and cells of fruit from silverleaf affected Fiesta apple (right). Scanning electron micrographs of cortical tissue of fruit from healthy and silverleaf affected Fiesta apple trees. The fruit was stored for 10 weeks at 2°C. The micrographs show the degraded and disrupted cells of fruit from silverleaf affected trees compared with the intact cells of fruit from healthy trees.

Only 55% of the fruit from silverleaf infected trees were sound. The defects included the following : bitter pit, brown heart, necrotic water core and fungal rots. No defects were found in the fruit from the healthy trees.

Therefore fruit harvested from silverleaf infected trees deteriorate quickly in store. This may be because the silverleaf fungus, Chondrostereum purpureum, produces the enzymes polygalacturonase and pectate lyase which are implicated in fruit and leaf damage.

The degree of fruit damage will be related to the level of foliar silvering - lightly silvered foliage will have little effect on fruit, but moderately and heavily silvered foliage will significantly affect fruit quality. If the tree recovers from a silverleaf infection, then the fruit quality should be acceptable. Fruit from moderately or heavily silvered trees should not be packed and coolstored - and certainly never exported.

Golden Queen peaches

Silverleaf also affected the yields of Golden Queen peaches. Yields from healthy Golden Queen trees averaged 59 kilograms, moderately silvered trees produced an average of 37 kilograms per tree and heavily silvered trees produced only 12 kilograms. The fewer fruit on the heavily silvered trees were heavier (healthy trees = 123 grams, moderately silvered trees = 130 grams, heavily silvered trees = 134 grams) and larger (63.5 versus 65.9 versus 66.4 mm respectively).

Brown rot incidence increased as the severity of the silverleaf infection increased. The peaches in this trial were not assessed further for quality at harvest and following storage.

What can the grower do ?

This is definitely a case when prevention is better than any cure. Care with orchard hygiene and tree pruning is important.

Trees which show symptoms of silverleaf need to be treated separately from the other trees in the block. Fruit from the silvered branches of moderate or severely infected trees should be segregated and not exported, although fruit picked amid healthy foliage will be unaffected. Trees with extensive foliar silvering are unsuitable for reworking.

Acknowledgment

HortResearch acknowledges the Foundation for Research, Science and Technology for financial support.

Source:
New for HortNET, August 1996. Edited and submitted by Helen Percy from "Silverleaf Disease, Tree Productivity and Fruit Quality" (Horticulture News, August 1996. Vol 18 No. 8 : p. 18).