Regarding the effect of weather conditions on the disease, it is known that the fungus grows most rapidly at a temperature of about 68 F., but will cause blossom infection at a temperature as low as 41 or as high as 86 . After it enters the blossom and is not so directly exposed to outside conditions, it develops rapidly even though outside air temperatures may be below the range at which it grows best.

Because the conidia will germinate only in the presence of moisture, little if any infection occurs when the weather is dry. A rain lasting only a few hours, however, will permit infection if the temperature is near the optimum for growth of the fungus. If temperature is below the optimum, a longer moist period is required for infection. The combined influence of temperature and moisture, in large measure, is responsible for the year-to-year fluctuations in the severity of the disease.

WHAT MEANS can be employed to control the disease? Let us recall that the disease can develop only if the fungus is present in the orchard or a nearby orchard, if the blossoms have reached an infectible stage, and if temperature and moisture are favorable for infection. Since no way is known to modify the temperature and moisture conditions in such a way as to prevent infection or prevent the blossoms from becoming susceptible to infection, the only approach by which we might control the disease is that of destroying the fungus or its conidia before they can initiate the disease. One way to do that is to cover the blossoms with a chemical that will kill the conidia that fall upon them.

We know that the blossoms become infectible as soon as they emerge from between the scales of the winter buds and remain so until after the petals fall. We must therefore give them a protective covering of a fungicide as soon as they emerge. Many field trials were necessary before it was learned just when to apply the spray, because the emergence and opening of the blossom is a progressive thing. The tip of the unopened blossom first appears, and as the stem elongates it is pushed out of the bud. Experiments conducted by a number of investigators showed that spraying before the blossoms emerged reduced infection to some extent but not enough to be of practical value. Spraying after the blossoms had emerged completely and the petals were unfolding often proved too late. Spraying just after the blossoms emerged gave the most satisfactory results. Even better control could be expected if the application was followed by another after the petals began to unfold. On the whole, such a schedule proves satisfactory once the grower becomes experienced in judging flower-bud development and in the mechanics of applying the spray. Nevertheless, even a two-application schedule proves inadequate in some years.

The most common cause of unsatisfactory results from spraying is a delay in giving the first treatment. Often rainy or windy weather at the time the spray should be applied interferes with spraying operations, and the fungus gains entrance to the blossoms. Once that occurs, spraying will not prevent the disease from developing. Such difficulties led to the development of another control method which does not require such critical timing of the spray treatments.

Monolinia laxa, it will be recalled, produces only one type of reproductive structure, the conidia. They develop in the tree on blighted twigs and blossoms and on the fruit, which the fungus occasionally attacks and which remains on the tree until spring. Early investigators recognized the desirability of eliminating the conidia and recommended that all blighted twigs and rotted fruit be removed when the trees are pruned in winter. Removal of the numerous blighted twigs proved impractical, however.

Chemical sprays to eliminate the conidia were developed. Monocalcium arsenite proves particularly effective for the purpose. When applied to dormant trees in mid-January, the compound destroys the conidium-bearing mats (sporodochia) present when the spray is applied and prevents their further development by killing the mycelium inside the twigs. One application of a preparation, containing 3 pounds of monocalcium arsenite to 100 gallons of water, commonly reduces sporodochia) development 95 to 98 percent and results in a decrease in blossom infection.

Since 1940 or so, growers in several localities have employed this treatment on apricots with comparatively little injury to the trees.

Some safety measures must be observed, however: Give the treatment only after the trees are completely dormant; in California the spray is applied from mid-January up to the time the flower buds begin to swell. Delay the treatment at least 2 weeks after the trees are pruned, or, better still, spray before pruning. Do not add spray oils to the preparation and avoid their use after the monocalcium arsenite treatment is given. It is probably unwise to give the treatment to trees in a low state of vegetative vigor. Yellowing of leaves accompanied by defoliation followed the application of monocalcium arsenite in the winter in one orchard.

Monocalcium arsenite is extremely poisonous and great care must betaken to avoid breathing or swallowing it:

ALTHOUGH monocalcium arsenite preparation does not materially injure the principal apricot varieties or the Santa Rosa and Wickson plums, it injures all almonds and some prunes. Consequently less injurious eradicative fungicides have been sought. Of about 75 other compounds that were tested, the sodium salts of the chlorophenols (particularly pentachlorophenol) eliminated the conidia most effectively. Sodium pentachlorophenate is destructive to the conidia present on the twigs when it is applied but is not highly effective in preventing their further development. Being very soluble, moreover, it is sometimes washed from the twigs by rain before its maximum effect on the conidia has been exerted. Under proper conditions, however, it destroys much of the conidia) inoculum. That in turn results in a significant decrease in the amount of blossom infection. Neither the eradicative nor the protective treatment alone satisfactorily controls the disease under all conditions. A combined eradicative-protective program is much more effective.

The protective fungicides most widely against the disease in California are the copper-containing materials, bordeaux mixture and the fixed coppers. Sulfur fungicides, although relatively effective under favorable conditions, cannot be used on apricots because of the "sulfur sickness" they produce in the tree. The newer types of fungicides, many of which are complex organic compounds, are being tested. Some show promise, but further tests are needed to evaluate their effectiveness.

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

Mr. Dunegan's discussion of peach brown rot appears on page 681.