Every grower is familiar with the tell-tale symptoms of silverleaf disease. Most would like to ignore it and hope that it goes away. Trees do recover naturally (particularly pipfruit) but silvering often returns when tress are stressed. Stone fruit generally declines (sometimes rapidly) and the grower is powerless to stop the disease.

Provided planting stock is free from silverleaf infection, plants can be kept healthy enough through careful attention to wounds at pruning.

Talking to growers and working in their orchards has convinced me that the life cycle of this disease is poorly understood. Also, many growers want to know which wound dressing to apply. This always puts me on the spot as it is difficult to give a simple answer. Wound dressings are expensive and many growers are rightly cynical about their effectiveness. Many make and apply their own concoctions, based on acrylic paint plus fungicide, often with disastrous results.

This article outlines the key points in the life cycle of the silverleaf fungus (Chondrostereum purpureum) which can make it such an effective wound coloniser. In particular, the environmental factors determining spore production will be outlined so that, safe periods for pruning can be identified. Also the wound infection process will be covered enabling identification of wounds at risk of infection. Finally, wound protection and wound protectants will be discussed.

Spore production - key facts

• Rainfall and high humidity determine the number of spores released from fruit bodies;

• Large numbers of spores are present on days unsuitable for the drying of washing (raining, drizzle, high humidity, foggy, calm, no wind or sun)

• Spore numbers drop rapidly on days favouring the drying of washing (strong winds, no rain, low humidity, sun). These conditions also favour the drying of wound dressings.

• To minimise the likelihood of wound infection, pruning should be done on fine days when relative humidity is below 70% and no rain has fallen for 24 hours. Strong, drying winds during pruning are a bonus.

Spore trapping studies have shown

1. Rainfall and relative humidity are the key factors governing spore release. Rainfall is essential since fruit bodies must increase their water content from 18% to 75% for spore formation. Spore release commences six hours following rain and continues as long as fruit bodies remain hydrated. Moist fruit bodies can obtain sufficient moisture from the atmosphere during periods of high relative humidity (>75% RH) to maintain spore discharge.

2. Duration of spore release is determined by relative humidity which in turn is governed by wind and radiation. Strong winds rapidly stop spore release leaving many spores attached to fruit bodies. These can be released later during calm periods should relative humidity rise. Spore trapping studies also detected low numbers of spores (50-1000 spores m³ air/hour) during dry periods. These were liberated from fruitbodies in damp shady locations.

3. Fruitbodies have tremendous capacity to produce spores, and will do so as long as there is sufficient moisture to keep them hydrated. In the trapping study, the longest period of continuous spore release was 106 hours. Had rain continued, spore release would have been maintained. The capacity of fruitbodies to produce spores is reduced after 12 months. Therefore, fruitbodies can be regarded as an important source of spores for one growing season.

4. Temperature has an important influence on spore production. Maximum spore production occurred 18 deg C then declined rapidly as temperature exceeded 20 deg C. Frosting did not kill fruitbodies but did delay spore production for a short period.

5. Light - more spores are released during darkness. In the field this effect is enhanced by rainfall and relative humidity which are also higher at night.

The life cycle of the silverleaf fungus is clearly adapted for wound infection during winter - fruitbodies form on dead branches during autumn. Their tremendous capacity for spore production is assisted by winter weather. That is, there are more rain days and mean daily rainfall and relative humidity is higher that at other times of the year. Less wind and lower sunshine reduce evaporation enabling fruitbodies to remain hydrated longer. By late spring/summer the aging fruitbodies have reduced capacity for sporulation and the weather is less favourable for sustained spore production. Because winter pruning coincides with a period of maximum spore production it is inevitable that unprotected wounds become infected.

Host infection - key facts

• the silverleaf fungus enters hosts as spores through wounds caused by pruning and wind damage.

• transfer by secateurs, pruning shears and root grafts is of no significance.

• because spores are windborne considerable distances (kms) the risks of infection can be reduced but not eliminated by good nursery hygiene (removal of dead infected trees, dead shelter).

• poplar and willow shelterbelts should be mulched and not left lying around in long grass to be colonised by silverleaf fungus. The long grass also keeps the fruitbodies hydrated which maintains spore production.

Wound penetration

Spores are smaller than starch grains and therefore can readily penetrate wood tissue. Most spores are retained within 10 mm of the wound surface, a few penetrate 30 mm, the occasional spore 70 mm. Once in wounds, spores are protected from the elements and can germinate and grow, obtaining food by digesting cell walls, starch grains etc. Once established, infection can advance rapidly (depending on host and time of year). In some willows and poplars infection can move more than 350 mm down the stem in 12 months. Field observations of infected trees (poplars, willows, apples, stonefruit) suggest infection moves more rapidly in stressed trees.

Wound diameter

Large wounds (>50 mm diam.) generally have more silverleaf infection (represented by brown stained wood) than medium wounds (20-50 mm diam.) or small wounds (10-200 mm diam.). This is due to a reduction in the inhibitory influence of the cambial layer and the greater surface area of large wounds. As a general precaution against silverleaf infection it would be wise to apply dressings to all wound larger than 10 mm in diameter. All wounds on the main stem/ trunk should be treated to prevent the entry of silverleaf which invariably results in more severe silvering than infection in peripheral branches.

Wound age

Wounds become more resistant to infection by the silverleaf fungus with time due to wound healing processes and competition from other micro-organisms which colonise wound tissues. The duration of the wound susceptibility period is unique to each host and therefore should be determined for that host. As a rule it is probably safe to assume that most wounds (made in winter) remain susceptible for at least one month. Wounds on S. matsudana x S. alba cv ‘Makara’ were still susceptible after 42 days. Brooks and Moore (1926) found that plum wounds exposed for a month were rarely infected whilst wounds exposed for three months were not infected. However, in most parts of New Zealand it is unlikely that unprotected wounds would escape infection for a month.

Seasonal susceptibility

To growers with crops restricted to winter pruning, it is of little consequence that their trees may be most resistant to silverleaf during summer. Field and laboratory studies have shown that willows are susceptible to silverleaf throughout the year (Spiers 1988). Field observations suggest that apples are also susceptible throughout the year. Plums (Brooks and Moore, 1926), peaches and nectarines (Dye 1967, 1972) are reported to be more resistant during summer. In contrast, eucalypts (Gadgil and Bawden, 1982) were more susceptible in spring and summer.

In the absence of detailed monthly silverleaf susceptibility data for fruit tree cultivars, it is safest to assume that they are susceptible to silverleaf infection throughout the year. Accordingly, all pruning wounds should be treated regardless of the season. Clearly, the risks of infection are reduced during hot, dry summers when inoculum levels are low.

Host susceptibility

Not all cultivars are equally susceptible to silverleaf infection. In fact, the most effective means to combat silverleaf is to breed and select cultivars with a high natural resistance to infection. We have bred but not yet released willow cultivars which are highly resistant to silverleaf infection. Silverleaf resistance is a criterion which should be considered in the selection of new fruit tree cultivars. It is almost impossible to protect pruning wounds of highly susceptible cultivars from silverleaf infection.

Inoculum concentration

There is no relationship between the number of spores landing on a wound and the depth of infection (brown staining) in that wound. This is not surprising since only two compatible basidiospores are necessary to initiate infection. High inoculum levels would not deposit spores deeper into wounds since most spores are retained within 10 mm of the wound surface.

The silverleaf fungus is an effective wound pathogen in winter because:

• spore numbers are at their highest;

• few spores are required to initiate infection;

• most wounds are large;

• all wounds are susceptible;

• host susceptibility is generally at its highest;

• wounds remain susceptible for a considerable period (at least a month);

• most wound dressings are ineffective.

Wound protection - key facts

• most wound dressings are cosmetic;

• it is almost impossible to keep the silverleaf fungus out of tree wounds;

• it is a waste of time applying wound dressings several days following pruning;

• in the absence of an effective wound dressing it is better to leave wounds untreated.

It is very difficult to keep silverleaf infection out of wounds. The problem is that the wood is so porous and is mainly dead tissue. This means that silverleaf spores can penetrate wood to varying depths and it is impossible to evenly coat tissues with fungicide. Because the tissues are dead, fungicides are not actively taken up and metabolised. Silverleaf spores simply bypass treated tissues and germinate. Fungicide treatment also eliminates many of the natural wound fungi which can prevent silverleaf infection.

The best means of protection is a physical barrier to prevent the entry of silverleaf spores. Again, this is not simple because wood has a high moisture content. For good adhesion, applied coatings must be sufficiently porous to allow movement of moisture vapour but not entry of silverleaf spores.

My search for the perfect wound dressing began in 1984 and has continued in an endless series of field trials since. Preliminary field, glasshouse and laboratory trials are the only way to test materials. The folly of laboratory studies is typified by results with 8 hydroxquinoline sulphate. This fungicide gave almost complete inhibition of mycelial growth and spore germination using filter paper discs on agar. The fungicides, dicholfluonid, captan and pyrocarbolid, performed poorly in the laboratory yet were the most effective in the field. Although time-consuming, the importance of thorough field trials cannot be underestimated. Products should not be marketed without adequate date to support label claims.

Testing method

Hybrid willows (S. matsudana x S. alba various cultivars) are used as the test species. They have advantages over other species in that they are: genetically uniform, readily propagated from cuttings, fast growing and highly susceptible to silver leaf.

Test plants are grown for two years (from cuttings) by which time they are 6 mm tall and up to 70-80 mm in diameter. They are cut 0.5 m above the ground during midwinter and a wound dressing is applied immediately. Twenty to thirty stems are used per treatment. Treatments are assessed (6-8-12 months later) by cutting wounds down the centre with a chainsaw and measuring the depth of brown staining caused by silverleaf infection. Depending on the willow cultivar, the amount of winter rainfall and the duration of the trial, untreated wounds generally have 80-140 mm of infection.

Trials

In an early trial (1984-85) various sealants including butyl rubber, tar bases and paint compounds were tested (Click here for trial results). The results show that none of the compounds inhibited silverleaf infection. The poor performance of acrylic paint plus captofol and Pruntect was not surprising in view of the failure of these dressings to protect poplar and willow stool bed nurseries. It is annoying to read reports still advocating the use of acrylic paint plus fungicide, even the application of copper fungicides, for wound protection.

The following season (1985-86), fungicides in water (at 3% a.i.) were applied to wounds (60-70 mm diam.). The fungicides were reapplied with a paint brush 2-3 times leaving a deposit on top of the cutting. The results were disappointing 6 months later when the trial was harvested.

Pyrocarbolid, captan, captafol, dichlofluanid and ditalimfos were significantly (P<0.05) better than other fungicides. However, no fungicide prevented the silverleaf fungus from establishing in wounds. The depth of silverleaf infection was greatest in wounds treated with copper-based fungicides. On the basis of this trial (and others) a tentative recommendation was made to catchment authorities to treat pruning wound in poplar and willow stoolbed nurseries with a mixture of dichlofluanid and captan (3% a.i. of each) applied in water. This treatment has been applied for a number of years and appears to be giving useful protection. However, it is not preventing silverleaf infection.

The search for more effective treatments continued and a further trial investigating the incorporation of fungicides into various bases was conducted (1987-88). Again, the results were disappointing. Clearly, the butyl rubber and other acrylic bases were inadequate. Other bases were tried and eventually an effective base was identified. Subsequent modification of this base by addition of further ingredients enabled the formulation of wound dressings which were trialed in 1988-89, 1990-91, and 1991-92. The results of the two later trials indicate that these new dressings give effective control of silverleaf infection.

During the 1992-93 growing season, three effective formulations in willow trials were trialed on apples, pears, cherries, peaches and plane trees. The treatments were assessed six months after application as described previously for willows. The results show that the dressings gave effective control of silverleaf on apples, pears and planes but were less effective on stone fruit.

The reasons for this are unknown and further research will be undertaken to formulate an effective dressing for the crop. These trials also show that genera react differently to wound dressing and a single formulation will not protect every crop. Nevertheless, the dressings gave significant (P>0.05) control of silverleaf on stone fruit in comparison to the untreated control wounds.

Biological wound protection

Fresh wounds are colonised by a natural succession of fungi and bacteria which can limit and prevent silverleaf infection from developing, particularly where spores of the silverleaf fungus do not infect wounds immediately. For this reason it is better not to apply ineffective dressings to wounds. Such products prevent the natural wound flora (fungi and bacteria) developing which facilitates the entry of silverleaf spores, enabling the development of more extensive silverleaf infection.

The possibility that naturally occurring organisms may provide control of silverleaf wound infection was investigated. Numerous fungi and bacteria isolated from old pruning wounds on willow shelterbelts were applied to fresh wounds on hybrid willows and assessed as described previously for wound dressings. Several isolates of Trichoderma (including the Swedish Bineb T isolate) were also tried. The inoculations were repeated several times with essentially similar results. None of the treatments prevented, or even reduced, silverleaf infection. The failure of Trichoderma treatments was surprising in view of favourable reports by Grosclaude (1970) and Corke et al (1979).

In conclusion

1. Don’t be fooled by the external appearance of a wound dressing. Many are purely cosmetic. Get a chainsaw and investigate the level of protection. Brown staining below the wound is generally silverleaf infection.

2. Don’t do it yourself when it comes to wound dressings. Never mix fungicides with acrylic paint. Never, never add copper to wounds or to paint.

3. It is better to leave wounds untreated than to apply home-made or substandard wound dressings. These exclude competing fungi, enhancing silverleaf.

4. Always apply wound dressings to wounds on the same day as the wound is made, allowing sufficient time for the dressing to dry. Remember spore numbers are higher during darkness.

5. Don’t prune on calm, damp, overcast days when spore loads are high. Conditions unfavourable to the drying of wound dressings (and household washing) are most conducive to silverleaf infection.

6. Don’t apply wound dressings several days after pruning. It is probably too late. Let the natural fungal flora combat the disease.

7. Apply dressings thickly to wounds. Large wounds deserve a second coat. Apply dressings with a small paint brush and lay the compound on.

8. Don’t thin dressings with water. Although dressings are expensive, it is false economy to thin them.

9. Be wary of both biological and conventional wound dressings. Ask for field (not laboratory) data to backup label claims.

10. Apply dressings to all wounds made on the main trunk. Infection of such wounds by Chondrostereum purpureum often results in extensive foliar silvering, particularly in susceptible varieties such as Fiesta.

REFERENCES

Brooks, F.T.; Moore, W.C. 1976: Silverleaf disease V. J. pomol. Hort. Sci. 5: 61-97.

Corke, A.T.K.; Hunter, T.; Sims, S.L; Andrews, L. 1979: Biological control of plant pathogens. Report of long Ashton Research Station for 1978.

Dye, M.H. 1967: Effect of pruning on silverleaf disease (Chondrostereum pupureum (Pers) Fr.7) and yield of peach and nectarine trees. N.Z J. Agric. Res. 10: 435-444.

Dye, M.H. 1972: Silverleaf disease of fruit trees. Bulletin 104. N.Z. M.A.F. 20p.

Gadgil, P.D.; Bawden, A.D. 1982: Infection of wounds in Eucalyptus delegatensis. N.Z. J. For. Sci; 11:262-270.

Grosclaude, C. 1970: Preliminary trials of biological protection of pruning wounds against Stereum purpureum. Annales de Phytopathogie 2: 507-516.

Shingo, A.L; Shortle, W. 1984: Wound dressings. Results of studies over thirteen years. Arboricultural Journal 8: 19-31.

Spiers, A.G. 1985: Factors affecting basidiospore release by Chondorstereum purpureum in New Zealand. Eur. J. For. Path. 15: 111-126.

Spiers, A.G. 1988: Factors affecting basidiospore release by Chondrostereum purpureum infection of Salix. Eur. J. For. Path. 18:257-278.

Spiers, A.G. 1988: Ultrastructural studies of basidial and basidiospore development and basidiospore release in Chondrostereum purpureum. Eur.J.For.Path. 18:367-381.

Woodgate-Jones, P.; Hunter, T. 1983: Integrated control of Chondrostereum purpureum in plum by treatment of pruning wounds. J. of Hort. Sci. 58: 491-495.

First published in The Orchardist, February 1994. Vol 67 No. 1 : 54-57.