First symptoms on apple and pear trees appear on foliage, and are characterized by poor terminal growth and small chlorotic, wilting leaves. Trees may also have a stunted aspect. By this time the development of the rotting in roots and crown is already in a well advanced stage. Upon removal of the bark, the internal tissues show a well-defined areas with a orange- to red-brown tinge. As the disease progresses, they enlarge and turn brown. The necrosis or rotting of the vascular tissues limits the supply of nutrients to the whole plant. General stress symptoms such as pale, wilting leaves and leaf drop ensue and stunted plant growth are due to that. Trees decline over several seasons and eventually die. Fruit rot can also occur, showing as dark-brown lesions that can affect the entire fruit. Fruit trees are susceptible to the rotting at different stages of maturation.
The symptoms are caused by the soil-borne fungus Phytophthora cactorum, which has an extremely large number of hosts. It thrives in wet soils, and can thus be a problem in low-lying areas, prone to flooding, or in moist field conditions. Warm periods also favors the production of spores and thus the infection. It attacks both apple and pear trees, but it is rarely a problem in the latter. The critical phase for infection is before the flowering stage. The major source of infection is fallen fruits emitting fungal spores or the introduction of infected transplants. Symptoms of crown and root rot occurs when the infection is below the soil line. Collar rot occurs above the soil line on the lower trunk. In both cases foliar symptoms are indicative of the rotting of the internal tissues of roots and the dysfunction of the vascular tissues.
To this day, we are not aware of any biological control method available against this disease. If you know of any successful method to reduce the incidence or he gravity of the symptoms, please contact us. However, fungicides containing copper can be used for the treatment of the infected trunk.
Always consider an integrated approach with preventive measures together with biological treatments if available. Commercial fungicides containing mefenoxam, etridiazol or fosetyl-aluminum can be used to decontaminate the soil, but are useless for the treatment of infected parts of the plant. Treatment with a combination of metalaxyl+mancozeb around the base of trees can also prevent the growth of P. cactorum in the trunks.