In this section:
Black Heart
Cause
Fungus Verticillium dahliae
Affected
Apricot is the worst affected, but the disease has been reported on other stone fruit.
Symptoms
The first evidence of the disease is the slow defoliation of affected limbs in early summer. The leaves wilt slightly, turn yellow and then fall. Leaf drop progresses upwards until, in severe cases the whole branch is defoliated except for a tuft of new leaves at the tip. Only part of the tree, or the whole, may be affected.
Flower and leaf buds on severely affected limbs dry out and fall off, and the branch dies. More commonly, the branch is not affected so severely and fruit sets but fails to develop. These branches, although not killed by the disease, are stunted and unproductive.
A distinguishing feature of the disease is that when cut off the laterals and limbs show a central ring or scattering of black dots in the inner wood.
Control
Firstly, when purchasing trees from nurseries, ensure they are free from black heart.
Black heart is caused by a fungus which can live in the soil for long periods without a host plant. Once soil has been infected, the disease can only be eradicated by soil fumigation and this is expensive and not always effective. Apricots should not be replanted into infected soil unless the planting site has been successfully fumigated. Soil tests can be carried out to detect whether the fungus is still present in the soil.
If replanting in an infected site, apricot and peach rootstocks should be avoided as they are very susceptible to black heart infection. Plum rootstocks are moderately to very resistant.
The fungus only spreads slowly through the soil, unless aided by cultivation equipment or irrigation water in the case of furrow irrigation. Therefore these practices should not be used in infected patches or the disease will spread to healthy trees.
Tomatoes are a common source of the disease. Apricots should not be planted on land which previously grew tomatoes and under no circumstances should tomatoes be intercropped with apricots. Strawberries and potatoes can also serve as hosts for the disease, along with a variety of weeds, particularly fat hen.
Where black heart is a problem, waterlogging should be avoided. Too frequent watering will aggravate the problem.
