Introduction

The silverleaf fungus, Chondrostereum purpureum, is an important pathogen of fruit trees, gaining entry through wounds.

A characteristic symptom of silverleaf infection in some plant species and cultivars is silvering of the foliage. C. purpureum may produce substances at the source of infection which are transported in the sap stream to the leaves and fruit of infected plants.

These substances degrade cell walls of leaves and fruit, while little apparent degradation of woody tissue occurs. This implies that the fungus targets pectic rather than cellulosic components of cell walls. Damage to leaf and fruit cell structure from silverleaf disease significantly affects the yield and fruit quality of infected trees. The extracellular fluid of C. purpureum grown in liquid cultures with and without inducible substrates was examined for the presence of enzymes contributing to cell wall degradation.

Method

Cultures were grown in liquid Beevers medium or standard minimal medium, with and without the addition of sawdust or polygalacturonate as inducing substrates. Cultures were examined after either 2 or 10 weeks of stationary growth at 20ºC.

The enzymes assayed for were :

Results

Cellulolytic enzymes

• There was little or no activity of the 3 cellulase-degrading enzymes tested for.
• Cellulase activity was not increased with the addition of sawdust to the medium.

Pectinolytic enzymes

• Polygalacturonase activity increased with time and was higher in cultures induced with polygalacturonate.
• High levels of pectate lyase were found in the older induced cultures. Little or no activity of pectin methyl esterase was found, and pectin lyase was never detected.

Figure 1 & 2: Healthy Gala apple leaf (left) and silvered Gala apple leaf (right). Vertical sections through healthy and silvered leaves of Gala apple. In silvered leaves, the palisade cells and the epidermal layer have become detached. The cell contents have become condensed, the chloroplasts and cell organelles have become moribund and the cells are highly vacuolate.

Figure 3: 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.

Enzyme activity in culture with time

Conclusions

• The lack of woody tissue degradation in silverleaf infected material is consistent with the low or negligible levels of cellulose-degrading enzymes produced by the fungus.

• The observed cell wall degradation of fruit and leaves from silverleaf affected trees is consistent with the production of some pectin-degrading enzymes. The disintegration of cell structure in fruit from silverleaf affected trees leads to a significant deterioration in fruit quality after storage.

• The pectin degrading enzymes found will be investigated further to find those that are specifically involved in leaf silvering and fruit deterioration.