146 DISEASES OF TROPICAL PLANTS CH . 



duced into the bark of another tree is not known. 

 Careful field studies in California have failed to connect 

 any insect or other agent with its transfer. Apparently 

 it must be carried in some purely accidental way since 

 isolated trees are often affected among healthy ones. 

 Also the disease may remain without apparent extension 

 for a considerable time. 



Remedies. The University of California recom- 

 mends the cutting of all knots from the trunk and 

 limbs with a strong knife or with a chisel and mallet, 

 and painting the wounds with a strong antiseptic. All 

 the knots should be pruned out from the smaller twigs. 

 Much foliage must not be removed in summer, but in 

 winter the trees may be very severely pruned. A 

 spray of Bordeaux or other good fungicide should be 

 given in winter or early spring, using care to cover the 

 trunk and limbs. 



It is believed that this will prove effective and 

 practical for the more resistant varieties since the olive 

 tree, growing under favourable conditions, is benefited 

 by regular and generous pruning. 



Fungus Root Rot (Armillaria mellea, (Vahl.) 

 Quel.). Armillaria mellea, or a very closely related 

 fungus, is rather common in California on many fruit 

 and ornamental trees. The fungus penetrates the 

 living bark of the roots and spreads out as fan-shaped, 

 felty, white mycelium, which rapidly kills the tissue. 

 Where the fungus pushes out into the soil, it assumes 

 the form of smooth black strands a little smaller than 

 the lead in a lead pencil. In December groups of large 

 smoky or tan-coloured toadstools are formed near the 

 affected trees. The fungus passes along the roots from 

 one tree to another, making a gradually enlarging area 

 in the orchard. 



Remedies. Olives succumb more slowly to this 

 disease than citrus fruits or stone fruits, but all affected 

 trees will probably die. All diseased trees should be 

 dug out and as many of the affected roots as possible 

 brought to the surface and burned. The land should 



