Dr. Smith demonstrated that effective control depended on the timing of the spray application at the beginning of the winter season. He summarized his findings somewhat as follows: Trees sprayed in December were free from coryneum blight and leaf curl; trees sprayed in January were free from leaf curl but affected somewhat by blight; trees sprayed in February and March were free from leaf curl, but severely affected by blight.

In our work at the California Agricultural Experiment Station the timing of the spray treatment in relation to leaf fall was studied closely. The foliage, of course, interferes with deposition of the spray on the twigs. It is desirable therefore to delay that treatment until the leaves are off the trees. Our results indicated that that can be done if the treatment is not delayed too long. The practice of some growers of spraying any time between leaf fall and late December is not to be recommended because severe twig infection may have preceded the treatment. in fact, a small amount of twig infection may occur before leaf fall. Between 1935 and 1941, for example, tests showed that infection before leaf fall occurred in two seasons out of the six, but in only one season was the amount sufficient to cause appreciable damage to the tree. Apparently, therefore, the practice of spraying soon after leaf fall is a safe one.

The single autumn treatment is expected to prevent twig infection throughout the dormant season, when heavy and prolonged rains occur. To be effective over such a long period, the fungicide deposit must have superior weather-resistant qualities. Our experience is that bordeaux mixture meets the requirements admirably; it is toxic to the conidia of Coryneum beijerinckii and it has excellent weather resistance as well. One-fourth or more of the copper deposited by bordeaux in autumn often is found on the twigs in spring. Few other fungicides can meet those requirements. A single treatment of lime-sulfur will protect the twigs for a short period but not throughout the winter. Some of the so-called fixed copper fungicides prove satisfactory if their weather resistance is increased by the addition of a sticker. Petroleum spray oil is used for the purpose. To obtain the maximum sticking qualities from such a supplement, the fungicide particles have to be incorporated in the oil phase of the emulsion. A stable oil-in-water emulsion is unsuitable, because the fungicide particles remain in the water phase and are wetted by the oil only after the water has evaporated from the spray deposit on the host. By selecting the proper emulsifier, however, conditions can be made that promote differential wetting of the fungicide particles by the oil.

We have considered so far the control of coryneum blight in peaches only. The procedure for controlling the disease in apricots is similar in that an autumn spray is applied. Its primary object is to prevent the fungus from attacking the dormant buds. Because leaf and fruit infection cause serious losses in apricots, however, it may be necessary to apply another spray in the spring.

Investigations in California and in Australia on the timing of this treatment demonstrated that leaf and fruit infection is prevented by spraying the trees just after the unopened blossoms emerged from the winter buds but before the petals unfolded. At times, a second spray soon after petal fall may be necessary but, by and large, the autumn treatment followed by the preblossom treatment is sufficient. As a preblossom treatment is given for the brown rot blossom blight disease in many parts of California, no additional expense is involved in following the two-treatment program.

Early attempts to control coryneum blight in almonds, following the same procedure as for peach, gave indifferent results. It soon became apparent in our tests, however, that coryneum blight on almonds is much less a wintertime disease than suspected. That is to say, twig and dormant bud infection, though present, are not so common as either to destroy appreciable numbers of twigs or to supply abundant conidia for infection. Then, too, infection of dormant buds apparently occurs more frequently in early spring than in winter. In any event, a bordeaux spray treatment just before the blossom buds begin to open materially reduces the number of buds infected. Moreover, such a treatment subsequently reduces the amount of flower and leaf-cluster blighting which can be a serious phase of coryneum blight on almonds. Experience indicated the need for an additional spray to prevent leaf infection in years when rainy weather extended into late spring. This spray is now applied soon after the trees are through blossoming.

The timing of the first treatment was later reexamined and it was decided to forego control of dormant bud infection in favor of more effective prevention of leaf-cluster and flower infection during the blossoming season. The first spray now is applied just after the blossom buds emerge from the winter buds but before petals unfold. A treatment at this stage of host development also aids in preventing brown rot blossom blight disease.

E. E. WILSON is professor of plant pathology in the University of California at Davis, where he has been engaged in studying fruit diseases since 1929.