USE OF HEAT TREATMENT AND CROSS PROTECTION TO IDENTIFY SOME SPECIFIC FRUIT TREE VIRUS DISEASES

Only symptoms can be used to identify these diseases, but different symptoms can be caused by the same virus.

The aim of heat therapy is to regenerate the infected cultivars. We can also use this technique to analyze virus disease complexes and to obtain single strains.

For example, when we treat an infected apple cultivar, we can obtain:

• virus-free plants
• plants still infected with 2 or 3 virus plus single strains of:

1. Chlorotic leaf spot virus (CLSV);
2. Spy decline virus (SDV);
3. Stem grooving virus (SGV).

However we cannot separate Spy decline, Pyronia decline, apple stem pitting, and quince sooty ringspot. These four symptoms are linked and probably caused by the same virus.

Cross protection can also be used to compare diverse symptoms.

But for each virus disease, it appears necessary to verify before the cross protection reality between mild and severe strains.

For example, we can study the cause of some peach stem pitting symptoms: NEPO virus or Peach latent mosaic virus or …

We can also consider comparing:

• Pear blister canker and Apple green crinkle;
• Apple russet ring and CLSV;
• Vein yellows and Spy decline …

Transmission of several fruit tree virus diseases to herbaceous plants has given negative results, therefore making it impossible to identify or purify these viruses.

Only the symptoms noted, often on one indicator only, have allowed to distinguish them. Such is the case for the majority of the virus diseases of apples and pears and also for different diseases affecting Prunus. This causes the use of a large quantity of indicators with sanitary controls at great expense.

The use of heat treatment in the last 10 years, to regenerate infected cultivars has also proved useful in the analysis of complexes in virus diseases.

Another technique, cross protection, has been successfully applied for the past 3 years at the Lanxade Center to detect Peach latent mosaic. The results show that cross protection can also prove useful in comparing or identifying certain virus diseases.

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