Various pathogens can reduce the vitality and productivity of Saxon oak forests and in some cases even endanger their survival. The predominant pathogens in the investigated conversion stands (seeds of plants of oaks under old trees of Scots pine) are Microsphaera alphitoides and Fusicoccum quercus, whose distribution varies depending on regions and sites. Their often considerable effects could be reduced significantly if better attention was paid in the cultivation of the stands and in the selection of the sites. Heterobasidion annosum often plays an important local role. Under certain circumstances (predisposition resulting from poor water and nutrient supplies, high infection pressure, the lack of natural antagonists) it can spread aggressively from old pine trees to young oaks. Fungi like micro-organisms which destroy fine roots (Pythium or Phytophthora ssp.) have a particularly damaging effect on the vitality of trees in old stands. Research has shown that weakened trees host a great number and variety of species. These are encouraged to spread e.g. by the use of heavy harvester and the resulting damage to roots. In pine forests however no Oomycota could be isolated from the soil. They cannot establish themselves in porous substrates (sand). On the other hand the examination of the fine roots of infected oaks showed that the pathogens, which probably originate from tree nurseries, survive in the plant tissue. In old oaks the vitality and value of the wood is reduced by various causative agents of wood rot. The percentage of old oaks suffering from root or stem rot could be significantly reduced by a more considerate use of harvesting techniques.
