366 PEAR GROWING IN CALIFORNIA. 



almonds, but under favorable conditions the trees contract the disease. 

 The fungus attacks oaks and other wild native trees and fruit trees 

 become infected when their roots come in contact with diseased roots 

 of other trees in the soil. The fungus lives on dead roots until they 

 have entirely decayed, consequently fruit trees may not contract the 

 trouble until years after they have been planted. 



The growth of the fungus on the bark of roots and crowns causes 

 decay and death of the affected parts. Girdling and death of infected 

 trees ultimately takes place. 



The nature of the disease does not favor a rapid spread throughout 

 an orchard, and it is usually detected by more or less circular areas 

 of dead trees. These areas are due to the spread of the disease from a 

 common center of infection by contact of good roots of surrounding 

 trees with those that already have the disease. 



Control. 



Professor W. T. Home* of the Department of Plant Pathology, Uni- 

 versity of California, has done more work than any one else in the 

 state to determine methods of control. First of all he recommends the 

 removal of all roots as far as possible from new land before planting to 

 trees. Growing alfalfa for a few years prior to planting is also sug- 

 gested. Experiments with citrus were conducted by Professor Home 

 in limiting the affected areas by trenching. No doubt this work could 

 be just as successfully done with pears as with oranges. We quote 

 from his Monthly Bulletin article, referred to in footnote, as follows: 



"One spot ditched was in an orange orchard in good, mellow soil, 

 trees good and more than ten years old. There were two dead trees and 

 two infected at the root but with the tops still fine. The ditch was made 

 3 to 3^ feet deep, no wider than necessary for digging. It seemed to 

 have cut all the roots. Infected roots could be readily recognized. As 

 finished it was believed that no diseased orange roots crossed outside 

 the ditch. A layer of tarred building paper of good quality was put 

 against one side of the ditch to prevent new roots from crossing back 

 into the diseased area. 



"After a little more than two years the ditch was reopened. The 

 building paper was worthless for stopping the roots, as they grew 

 through it very readily. The rest of the experiment was highly encour- 

 aging. In repeated cases a root from which a piece had been cut out 

 could be recognized unmistakably on the two sides of the ditch. The 

 piece within the diseased area would be in an advanced condition of 

 decay with the fungus, while the end toward the unaffected tree and 

 outside the diseased area would be entirely unattacked and putting out 

 numerous new roots. There could be no reason to doubt that if the 

 ditch had not been made the fungus would surely have followed the 

 root and there would have been no hope of saving the tree. Around this 

 area, which included four diseased trees, no less than five good trees 

 were saved from infection. If once opening the ditch will save the 

 sound tree from infection for two years, there is no reason why the 

 thing can not be done again in the same place and the spot permanently 

 restrained to its present area/' 



'Monthly Bulletin, State Commission of Horticulture, Vol. Ill, No. 7. 



143 



