The plum in common with other stone-fruits often suffers from an excessive flow of gum, for which trouble the name gummosis[168] is now generally applied. The disease is to be found wherever plums are grown but it is much more destructive on the Pacific than on the Atlantic seaboard. So far as is now known gummosis is secondary to injuries caused by fungi, bacteria, insects, frost, sunscald, and mechanical agencies. The disease is least common in species and varieties having hard wood; on trees on soils favoring the maturity of wood; under conditions where sun and frost are not injurious; and, obviously, in orchards where by good care the primary causes of gumming are kept out. Stewart[169] has recorded an interesting case of gum-pockets in the fruit, but could assign no cause.
Mechanical injuries from the sun, frost and hail are troubles with which nearly all plum-growers must contend at one time or another. In this region the Reine Claude and Triflora plums suffer much from sunscald but none are wholly immune, though Lombard is possibly most so. These injuries from the elements of weather are often mistaken for diseases, and are so often followed by fungal parasites and insects as to make it difficult to distinguish the primary from the secondary trouble. Low-heading of the trees is the best preventive of these trunk injuries.
Plums are somewhat subject to attacks of the well-known peach scab[170] (Cladosporium carpophilum Thumen). The scab appears in numerous, small, sooty, circular spots of brownish color, often confined to one side of a fruit but in other cases distributed over the whole surface. None of the cultivated species are free from the disease but the Munsoniana, and Hortulana varieties are most susceptible to it. Pear blight,[171] (Bacillus amylovorus (Burrill) DeToni) commonly thought of as a disease of the pear and apple has been found on various plums, and the yellows of the peach, cause unknown, is often quite destructive to Triflora plums. According to Smith the peach rosette,[172] cause unknown, attacks both wild and cultivated plums in the South and is quickly fatal. The disease was prevalent on the wild Angustifolias, on two varieties of Triflora, Kelsey and Botan, but the observer had not seen rosette on varieties of Domestica.
Waugh describes a trouble which he calls “flyspeck fungus”[173] found on fruits sent from the Southern States, in which small areas are thickly dotted with black spots; also a fruit-spot on plums from Texas caused, as he states, by an undetermined Phoma.[174] Starnes of Georgia describes a malady of the Triflora plums called “wilt,”[175] cause unknown, which he states is the most serious obstacle to the culture of this plum in the South. In this peculiar disease the foliage passes directly from a green, healthy state into a wilted and then parched condition, the death warrant being signed when a tree is once affected. In Oregon and Washington the Italian Prune is subject to a leaf-curl[176] which begins in mid-summer and curls the leaves conduplicately without withering but shriveling somewhat. As the season advances the leaves turn yellow and many of them drop. Neither cause nor cure is known. Smith described a plum-blight[177] of native plums in Georgia which “destroys large branches or even whole trees in mid-summer in the course of a few weeks.”
INSECTS.
Cultivated plums furnish food for a great number of insects. Many of the destructive insect pests of the several cultivated species of Prunus are known to have come from the wild plants of the genus, but others, and possibly the majority, come from over the seas. No less than forty species of insects may be enumerated as pests of the plum and many more can be counted as occasional parasites on one or another of the species. Of the formidable pests the plum curculio is probably the most troublesome. The plum curculio[178] (Conotrachelus nenuphar Herbst) is a rough, grayish snout-beetle somewhat less than a quarter of an inch in length, an insect so familiar to fruit-growers as hardly to need a description. The female beetle pierces the skin of the young plums and places an egg in the puncture. About this cavity she gouges out a crescent-shaped trench, the puncture and trench making the “star and crescent” of the Ottoman Empire, hence the common name of the beetle, “The Little Turk.” The egg-laying process may be repeated in a number of fruits and from each egg a larva hatches within a week and burrows to the stone, making a wormy fruit. Most of the infested plums drop. In years past plum-growers relied upon jarring the beetles from the trees in the early morning, but the treatment was too expensive, and poisoning with an arsenate is now the chief means of combating the pest. Rubbish and vegetation offer hiding places for the insects and hence cultivated orchards are more free from curculio. Thin skinned varieties are damaged most by the insect but there are no “curculio-proof” plums.
A larger snout-beetle than the curculio, the plum gouger[179] (Anthonomus scutellaris LeConte), occasionally does much damage to plums. The work of the gouger may be told from that of the curculio by the absence of the crescent cut about the puncture made for the egg, and from the fact that the larvæ of this pest chiefly infest the stone and those of the other insect the flesh of the plum. The remedies are the same for the two insects though the gouger is more easily destroyed.
Among the several borers which are more or less destructive to species of Rosaceae only the peach borer[180] (Sanninoidea exitiosa Say.) may be counted as a troublesome pest of the plum. The larvæ of this insect are frequently to be found in both wild and cultivated plum trees and must be combated in nearly all plum orchards east of the Rocky Mountains. The prevention of the work of the borer is best accomplished by thorough cultivation, the use of coverings of tar and poisonous washes and mounding the trees. Destruction is effectively carried out only by digging out the borer with knife or wire. The lesser peach borer[181] (Sesia pictipes Grote & Robinson) attacks the plum and in New York has been found particularly injurious to the Wickson plums. The flat-headed apple tree borer[182] (Chrysobothris femorata Fabricius) is frequently found in the wood of wild plums and is sometimes a pest of the several cultivated species. It is treated as is the peach borer. The shot-hole borer[183] (Eccoptogaster rugulosus Ratzeburg) a diminutive insect which deposits its eggs in the trunks or large branches of various members of the genus Prunus, may be regarded as an effect rather than a cause of disease, for it seldom injures perfectly healthy plum trees. The peach bark-beetle[184] (Phlœotribus liminaris Harris) is somewhat similar in its work to the shot-hole borer and like it is found for most part only in diseased and decrepit trees.
The plum aphis[185] (Aphis prunifolii Fitch) is sometimes very destructive to varieties of the native plums, especially the Americanas, and occasionally injures or even kills the young trees of the Domestica sorts. It is not a formidable foe in New York, and it is the exception when trees must be treated for it, the treatment being any of the contact solutions used against sucking insects. The cherry aphis[186] (Myzus cerasi Linnaeus) and the green peach aphis[187] (Myzus persicæ Sulzer) are much less common than the plum aphis on plum trees, but are sometimes abundant on foliage of this fruit and are combated in the same way as the more common aphis. Gillette enumerates two other aphids as attacking the plum in Colorado—the rusty brown plum louse[188] (Aphis setariæ Thomas) and the mealy plum louse[189] (Hyalopterus arundinis Fabricius).
Several scale insects infest the plum. Chief of these is the dreaded San José scale[190] (Aspidiotus perniciosus Comstock) known and feared by all fruit-growers in the United States. The lime and sulphur solution is now the most common and probably the most effective spray for this insect. The European fruit lecanium[191] (Lecanium corni Bouche) occasionally does a great deal of damage in New York and now and then destroys the whole crop in an orchard. The winter treatment for San José scale is used to control this pest, but usually such treatment is supplemented by a summer spray about July first with such contact sprays as whale oil soap and kerosene emulsion. Of the other scales[192] which feed upon plums and now and then become pestiferous the following may be named: The fruit pulvinaria (Pulvinaria amygdali Cockerell), the mealy bug (Pseudococcus longispinus Targioni), the scurfy scale (Chionaspis furfura Fitch.), the West Indian peach scale (Aulacaspis pentagona Targioni), the Putnam scale (Aspidiotus ancylus Putnam), the cherry scale (Aspidiotus forbesi Johnson), the walnut scale (Aspidiotus juglans-regiæ Comstock), Howard’s scale (Aspidiotus howardii Cockerell), the European fruit scale (Aspidiotus ostreæformis Curtis), the red scale of California (Chrysomphalus aurantii Maskell), the oyster-shell scale (Lepidosaphes ulmi Linnaeus), and the soft scale (Coccus hesperidum Linnaeus).