Conclusion of Experiments. As a result of these experiments it can be stated that frog-eye leaf spots, initial infection and enlargements, are caused by Sphaeropsis malorum. Alternaria mali is a saprophyte spreading through the spot rapidly behind Sphæropsis and rarely becoming a facultative parasite under conditions of excessive moisture. Phyllosticta pirina, Phyllosticta limitata and Coniothyrium pirinum are saprophytes able to grow and fruit on the dead spots but not able to produce enlargement. The light gray color of some round. frog-eye spots which have ceased to enlarge is due to infection with P. pirina. (Fig. 4.) Coryneum, Hendersonia, Pestalozzia, and Illosporium, which appear on the dead tissue late in the season, are saprophytes. THE RELATION OF FROG-EYE LEAF SPOT TO BLACK ROT AND CANKER. The frog-eye spot is caused by Sphaeropsis malorum, the same organism as that which produces black rot of apple fruits and cankers on trunks and branches. In order to combat this disease it is necessary to understand the relation between its different forms. The control of one involves the control of the others. Fig. 5.-The Black Rot of Apples. The small mummy furnished Sphæropsis spores for the infection of the apple below it which is completely rotted and on which Sphæropsis pycnidia are almost ripe. The large apple shows Black Rot spreading from an infection of the calyx. The leaves below these apples were badly infected with Frog-Eye Leaf Spot. Photographed August 6. Black rot may appear on apples at any age but usually is first noticeable when the fruits are about half grown. Minute purple discolorations are the first indications of the disease. On spreading, these form circular, depressed spots, under which the tissue is brown and practically tasteless. (Fig. 5.) Subsequent spreading consists in the addition of brown concentric rings about the young spot and becomes especially rapid after ripening begins. The brown color of the skin of the diseased portion of the apple has given this disease the name "brown rot" in some localities. It is easily distinguished from "bitter rot in that it lacks the bitter taste. Apples affected with black rot usually fall when about half the tissue has been destroyed. On the ground they continue to rot and finally dry up into hard black mummies. (Fig. 5.) Others which remain on the tree do likewise and may even hang on all winter. During the autumn and winter, the Sphæropsis in these mummies begins to produce spores which are mature and ready for dissemination at the beginning of the growing period the next spring. Black rot cankers are formed on trunks, branches and twigs in much the same way. Frequently twigs killed by blight are infected by Sphæropsis and develop cankers. These cankers produce myriads of spores each spring. It is these cankers and mummied apples which enable the disease to live through the winter and which furnish the Sphæropsis spores for the infection of apple leaves and the production of frog-eye spot. The indications are that infected leaves wintered on the ground do not constitute an important source of infection for the next year's foliage. Badly infected. leaves which had passed the winter under trees in the orchard and in wire baskets out of doors have been thoroughly examined during the winter and spring and only a very few Sphæropsis spores could be found. Unusually severe infections of frog-eye, have, however, been repeatedly traced to cankers, and mummies or dried-up rotten apples that had remained on the tree over winter. THE CONDITIONS WHICH INFLUENCE INFECTION. The controlling factors of infection are chiefly three: 1.-ABUNDANCE OF INFECTIVE MATERIAL.-Cankered limbs and twigs and mummied apples are the sources of the Sphæropsis spores, the dissemination of which begins when the first warm rains in spring soften the pycnidia and cause the exudation of spores. This process begins usually in March but as far as infection is concerned is of no importance until the apple trees leaf out. The careful pruning out of all dead or cankered wood and the destruction of mummies prior to this time will greatly reduce the amount of infective material. 2.-ABUNDANCE OF MOISTURE.-The spores of Sphæropsis are capable of germination only in the presence of moisture. Rain or dew is therefore essential to successful infection. Several season's observations show that heavy rainfall in May and June is always accompanied by severe infection with frog-eye leaf spot, while the reverse is true when spring rainfall is slight. However, there is always enough rain in spring to make this disease. a serious consideration. 3. SUSCEPTIBILITY OF HOSTS.-No variety of apple is immune to frogeye but some are more susceptible than others. Of the very susceptible varieties, Ben Davis, Winesap, Arkansas, Baldwin and Jonathan are noteworthy. Early Harvest, Grimes, Yellow Newtown, (Albemarle Pippin), Gano and York Imperial are among the least susceptible. Artificial inoculations on York Imperial leaves show that old leaves are as susceptible as young ones; in fact, they appear to be more so. Old and young leaves were inoculated at the same time and kept under the same conditions, the old leaves developed five times as many spots as the young ones. When the parasite has gained entrance to the host plant there is nothing which can be done to mitigate or stop the injury which it may cause. The control of this fungus can be made successful only by the prevention of infection. SPRAYING EXPERIMENTS FOR THE CONTROL During the years 1910 to 1914, inclusive, extensive spraying experiments for the control of frog-eye leaf spot have been conducted in various parts of Virginia. Orchards which were badly diseased in previous years were selected for these tests. From fifty to eighty trees of four varieties, York Imperial, Winesap, Ben Davis and Jonathan, were used each year. Unsprayed trees of each variety were retained as checks. The sprays tested were: (1) Lime-sulphur (1 gallons commercial solution, testing 32° Beaume', to 50 gallons water). (2) Bordeaux mixture (4-5-50). (3) Copper-lime-sulphur, (1-50, plus 2 lbs. copper sulphate). (4) Iron Bordeaux, (4-5-50, plus 3 lbs. iron sulphate). Beginning just after the petals had fallen from the apple blossoms these sprays were applied three to five times at intervals of 10 to 21 days. On the basis of three applications, 14 days apart, beginning when the petals had all fallen, the average estimated efficiency of each spray material in all the experiments conducted during the five years is numerically expressed in Table VI. TABLE VI.-Relative Efficiency of Spray Material in Controlling Lime-sulphur Bordeaux mixture.. Iron Bordeaux.. Copper-lime-sulphur .95% 85% .85% ..85% In some instances all of these sprays were 100 percent efficient, but ordinary lime-sulphur holds the highest average. The other sprays, all of which contain copper-sulphate, have under certain weather conditions, produced a type of spray injury which in the early part of the summer is almost indistinguishable from young frog-eye leaf spot. These spots, however, remain small and circular throughout the summer. Saprophytic fungi may be isolated from them but Sphæropsis is absent. The initial injury is without a doubt caused by copper compounds and the dead tissue is later infected by saprophytes, principally Phyllosticta and Alternaria. The close resemblance of this type of spray injury to the true frog-eye spot may account in part for the low efficiency figures for the copper containing mixtures, but since the injury is similar in its effect on the tree, the figures still represent true comparative values. In all of the experiments unsprayed check trees lost from 25 to 95 percent of their foliage before September, while adjacent sprayed trees retained full coverings of leaves and matured excellent crops of fruit. RECOMMENDATIONS FOR THE CONTROL OF THE DISEASE. The control of frog-eye leaf spot and incidently the control of black rot and Sphæropsis canker, is comparatively simple. It involves (1) the destruction of the sources of infection, viz., cankers and mummies or driedup diseased apples remaining on the tree over winter, and (2) spraying for the prevention of infection. 1.-Destruction of the Sources of Infection. Cankers and mummies can best be eradicated in winter when the leaves are off the trees. All dead or cankered wood and mummies should be removed from the trees and burned. This can best be done in the regular course of pruning practice. 2.-Spraying to Prevent Frog-Eye Leaf Spot. We recommend the same treatments for the prevention of this leaf spot that are used by fruit growers to control apple scab. Spraying with Bordeaux or Lime-sulphur (summer strength), before the bloom opens, supplemented by one or two later treatments at intervals of from ten days to two weeks, will prevent the development of either apple scab or frog-eye leaf spot. By the use of three or four applications of lime-sulphur at intervals of ten to fourteen days, it is possible to almost entirely eliminate both frog-eye leaf spot and black rot, and to keep them under control from season to season. SUMMARY. The frog-eye leaf spot, though usually considered as one of the lesser important apple diseases. is a cause of serious losses in neglected orchards. The small circular brown spots, which are the first evidence of the disease, appear on the leaves about the first of May. The later development of the spots is more or less irregular, though usually they gradually enlarge as the season advances. When infection is severe many of the leaves turn yellow and fall off by midsummer. The injury to and loss of the foliage results in a reduction of the fruit crop, and tends to lower the vitality of the tree. For a number of years there has been some uncertainty as to what fungus was responsible for the frog-eye spot. This has been due to the fact that a number of different organisms have been found closely associated in the diseased portion of the leaf. Investigations have shown that both the initial infection and secondary enlargement of frog-eye spot are due to Sphaeropsis malorum. Sphæropsis is constantly present in very young spots; after the spot is a few weeks old the spores of many species of fungi may be found on its surface. The fungus which causes the frog-eye spot is already well known. In addition to this leaf spot, Sphaeropsis malorum is also responsible for a canker that occurs on the trunk and branches of the apple tree, and for black rot of the fruit of apples, pears and quinces. Spraying experiments, which covered a period of five years, have shown that the regular applications of lime-sulphur spray or Bordeaux mixture as employed by fruit growers to control apple scab and black rot, will also prevent damage by frog-eye leaf spot. |