THE FUNGUS that causes leaf spot on sour cherry and sweet cherry in the United States conforms generally with the description of Coccomyces hiemalis. Probably the fungus is the only common one on Prunus mahaleb.
B. B. Higgins, of the Cornell University Agricultural Experiment Station in New York, in 1913 and 1914 divided the various isolates or collections of Coccomyces that he studied into three species based on both morphologic and host-range differences: Coccomyces hiemalis on sweet cherry (Prunus avium), sour cherry (P. cerasus), and pin cherry (P. pennsylvanica); Coccomyces prunophorae on the plums Prunus americans, P. domestics, and P. insititia; and Coccomyces lutescens on the wild black cherry (Prunus serotina), the chokecherry (P. virginiana), and Prunus mahaleb.
G. W. Keitt in Wisconsin in 1918 Published the results of more than 1,000 cross-inoculation tests and added others in 1937. He used isolates of Coccomyces from all three of the groups Of cherries and plums set up by Higgins. In no case did the Coccomyces isolate from any two Prunus species show exactly the same host relationships. Further, the same isolate commonly infected different hosts with different degrees of severity, varying from slight flecking to abundant production of typical leaf spots. He tentatively grouped the fungi as follows according to the plant from which they were obtained. Prunus cerasus, P. avium, P. mahaleb, and P. pennsylvanica; P. domestics; P. virginiana; and P. serotina. Prunus mahaleb was susceptible to isolates of all four groups. P. cerasus was infected only by isolates from Group 1.
J. B. Mowry, of the Indiana Agricultural Experiment Station, reported in 1951 the inoculation of 66 species, varieties, and hybrids of Prunus with single-spore cultures of Coccomyces. He added Prunus fruticosa to Keitt's Group 1. He obtained infections on sour cherry and Prunus mahaleb with the isolate from P. serotina, and on both sweet and sour cherry with the isolate from P. pennsylvanica. Seedlings of P. cerasus, P. insititia, P. mahaleb, and P. tenella were susceptible to most isolates tested. Seedlings of P. besseyi, P. japonica, P. pumda, P. persica, P. salicina, P. serotina, P. spinosa, and P. virginiana were susceptible to relatively few isolates. Seedlings of P. glandulosa and P. maritima were resistant to all seven isolates.
While future work may be needed to clear up some aspects of this situation it seems clear that the Coccomyces fungi which cause leaf spot on the cultivated cherries, Prunus cerasus, P. avium, and P. mahaleb, form a group which conforms in general with Higgins' description of Coccomyces hiemalis. It is evident from the work I have described and that of R. O. Magic in Wisconsin in 1935 that this fungus is able to cause leaf spot on several other Prunus species, including the plums, under more or less ideal conditions. No evidence has been found that C. lutescens is of any significant importance on cherries. There is no known evidence that the common wild cherries, P. pennsylvanica and P. serotina, are of any importance as a source of the leaf spot fungus in sour cherry orchards.
Coccomyces hiemalis belongs to the order Phacidiales of a group of fungi referred to as Ascomycetes because they bear the spores of the perfect or sexual stage in a club-shaped organ called an ascus. Dr. Higgins first described it in 1913. He found the perfect stage of the fungus on the leaves of sweet cherry, P. avium, and showed that it was the fungus that previously had been called Cylindrosporium.
Coccomyces hiemalis passes the winter in the old leaves on the ground as a partly formed, round or somewhat elongated, dark-colored fruit body, which normally extends from the lower to the upper epidermis of the leaf but remains covered above and below by the epidermis.
The fruit body, or stroma, begins to swell toward the lower leaf surface during the first warm days in the spring. Club-shaped asci then form within the stroma. There follows the formation of eight two-celled ascospores within each ascus. As the asci enlarge rapidly within the stroma, the covering of the fruit body is lifted until it ruptures. The ascospores within the asci mature shortly afterwards normally when the sour cherry is in the pink, or early-bloom, stage of growth. The ascospores are discharged through the end of the ascus in wet weather and are carried upward by wind. If they lodge on a susceptible leaf under favorable conditions, the ascospores germinate and leaf spot results in 1 to 2 weeks. Penetration of the leaf by the germ tube from the ascospore occurs through the stomata of the leaf.
After the fungus invades the leaf, a disk-shaped mass of fungus mycelium is formed beneath the epidermis of the leaf. Secondary spores, the conidia, are borne on the surface of this mass.
When they have accumulated in sufficient numbers, the epidermis of the leaf is ruptured and the conidia appear in a pink to whitish-pink mass. The conidia, or summer spores, are borne in large numbers and are spread from leaf to leaf by water. The rapid spread of leaf spot in the summer and fall is usually due to the rapid increase and spread of the fungus by means of repeated generations of conidia throughout the summer and fall.
Besides the regular, or normal, ascospores and conidia, the fungus produces conidia in the overwintering fruit body after the ascospores are discharged in the spring. The conidia cause leaf spot if placed on a susceptible leaf under favorable conditions, but may be of little significance in the normal reproduction of the fungus. The fungus also produces small spores, called microconidia, on the leaves in fall. Their function in the reproduction of the fungus is unknown.
AN APPROACH to the problem of control of a disease of this type is based on the knowledge that we are dealing with two plants, the cherry tree and the fungus Coccomyces in this case, both of which have their normal manner of development and sensitivity to various influences. The modern orchard sets up a nearly ideal situation for the reproduction of both plants. Our purpose is to interfere in some way with the reproduction of the fungus without seriously injuring the cherry tree.
A healthy sour cherry blooms while the leaves are still small. The leaves are folded along the midrib while small and begin to unfold during the latter part of the blooming period, usually while the flower petals are falling. The growth of the spur leaves is rapidly completed after petal fall, but growth of new leaves on the terminals continues until midsummer.
The leaf spot fungus rarely infects the very young leaves, apparently because the stomata through which the fungus enters the leaf are not mature until about the time the leaf unfolds. Once unfolded, however, the leaves are susceptible to infection throughout the summer and fall.
As the fungus overwinters in the old leaves on the ground, any factor that reduces the prevalence of the fungus One year will reduce the likelihood of serious trouble with the disease the following spring. Thus few infections one year make it less likely that leaf spot will be a problem the following year. Too much faith in this fact, however, has led to trouble. The fungus often spreads rapidly in the fall and may overwinter at a high population level although it was not a problem earlier in the growing season. Too, if the ascospores are abundant enough to establish the fungus on the tree in the spring, it may spread rapidly in wet season.
In dooryard or garden trees outside of orchard districts, the fungus can be destroyed by raking and burning the old leaves on the ground in fall and winter. That work is not practical in commercial orchards because of the labor involved, but part of the same purpose can be served by disking or plowing the old leaves under before the ascospores mature in the spring or by spraying the old leaves with one or more chemicals that destroy the fungus in them.
Clean cultivation of the orchard would be desirable if the sole aim were the destruction of the leaf spot fungus, but that is often not practical because of the cost, the shortage of labor, and the desirability of maintaining some sort of trashy soil cover in the orchard. A considerable reduction in the population of the fungus can be obtained merely by disking both ways along the tree rows in the orchard.
Good results have been obtained in reducing the production of ascospores in the old leaves on the ground by means of fungicidal ground sprays. The most common spray material used for the purpose has been the sodium salt of dinitro-o-cresol, sold under the trade names of Elgetol and Krenite and used at one-half gallon of the paste to 100 gallons of spray and applied at the rate of about 500 gallons the acre. The mixture seems to have special merit in orchards where cultivation is not feasible, but it has not been generally accepted in commercial practice. The objections to it are usually the extra cash outlay required, the shortage of labor, and inability to reduce safely the number of summer sprays in districts where neighboring growers allow the fungus to overwinter undisturbed.
THE DIFFERENCE in sensitivity of the tree and the leaf spot fungus to the action of chemicals often permits a high degree of control with sprays applied to the tree. Such sprays are now the principal means of control in commercial orchards and nurseries.
The time when the first fungicidal spray is needed depends on the presence of leaves large enough to be susceptible, the presence of mature ascospores of the fungus, the presence of moisture for a sufficient length of time to permit infection, and a temperature at which the fungus will grow.
All those conditions normally are met about the time the flower petals fall, and most spray schedules call for the first application then. That has been satisfactory when the population of the fungus is at a reasonable level and the ascospore-induced infections are few. At times, however, the petals are slow in falling and leaf growth is rapid during late bloom. Such a situation, coupled with a high population of the fungus and wet weather near petal fall, may permit many infections. The application of a spray just as the first flowers opened on sour cherry in Pennsylvania in 1947 reduced the percentage of infected leaves 3 weeks later to 1.9, compared to 24.5 with sprays started at petal fall and 92.7 on trees not sprayed at all.
Contrary situations do occur. Few or no infections may occur until a month or more after petal fall, perhaps because of a scarcity of the fungus.
Following the fungicidal spray at petal fall, additional sprays are applied as needed. On sour cherry that normally means a second spray about 10 days after the first, two sprays in June, and the last just after the fruit is picked in July. More sprays timed at closer intervals and applied before the long rain periods during the season are often needed on rapidly growing nursery trees, on orchard trees growing in areas with a long growing season, and in places where an attempt is to be made to control a high population of the fungus without the use of a copper fungicide. A combination of all these factors has led to the general use of eight or nine sprays each season in south central Pennsylvania.