Du
18 April
au
31 December 2016

Chancres associés à Phoma exigua sur semis de frêne en pépinières forestières

Cankers on ash tree seedlings in forest nurseries, caused by Phoma exigua

Context

Canker of ash tree seedlings (Fraxinus sp.) caused by Phoma exigua first appeared in Belgian nurseries at the end of the 1990s. Several cases have been reported since then, in both the northern and southern parts of the country. These cankers usually appear as cracked tissues (fig. 1), but can also look like small depressions or sometimes surround the whole stem. Developing cankers can coalesce at a later state into large affected areas, causing the plant to break at the slightest shock. The callusing of the cankers is sometimes visible when symptoms are mild. Similar symptoms have been observed on ash tree seedlings elsewhere in Europe, notably in France and in Great Britain. The causal agent is not considered to be a weak parasite: both healthy and weakened seedlings can be infested, but seedlings less than 3 years old are the most sensitive. Humidity favours the establishment of the disease. Before this study, no fungicide was registered in Belgium to control this disease. This lack of registered products is common in horticultural crops whose market share is too limited to interest crop protection companies. This poses a problem for the nurseries sector which generates high added-value products.

Objectives

In recent years there have been several cases reported in Belgium of canker caused by Phoma exigua leading to a total loss of the affected crops, resulting in substantial profit loss. The main goal of this study was to evaluate the effectiveness of various fungicides with the potential to control Phoma exigua in order to draw up a file on extending the use of one of these products (use extension file) for the Federal Public Service, Public Health, Food Chain Safety and Environment.

Description of tasks

1)Collection of isolates Phoma exigua strains isolated from cankers of ash tree seedlings from several Belgian nurseries are being maintained in a collection and used as material for inoculation tests or in vitro trials. 2)Koch’s postulation In the past, Phoma exigua was associated with stem canker on Fraxinus sp. in nurseries in various European countries, but pathogenicity was never established. Inoculation and re-isolation tests were therefore undertaken to demonstrate this pathogenicity. 3)Evaluation of fungicide effectiveness The project is evaluating the effectiveness of fungicides against stem canker of ash tree seedlings, caused by P. exigua. In vitro tests were performed to evaluate the ability of the fungicides to inhibit mycelium growth and spore germination. The extent to which they reduced the symptoms was estimated in a field trial. 4)Differentiation of Phoma exigua varieties In the P. exigua complex, several host-specific varieties (e.g., var. populi on poplar and var. forsythiae on forsythia) have been described. These varieties can be identified by their cultural characteristics and through molecular analysis (notably, through AFLP fingerprints). Tests will be performed to establish whether the P. exigua strains isolated from cankers on Fraxinus sp. represent a separate variety.

Expected results

The project aims at improving the knowledge of P. exigua and drawing up a use extension file of a fungicide for the control of canker caused by P. exigua in forest nurseries.

Contribution

Use extension file for Topsin M 500 SC, thiophanate-methyl (file number 4477)

Partners

Several CRA-W's Research Units take part in this project

Funding

  • CRA-W - Walloon Agricultural Research Centre

Team