The plums of New York

by U. P. Hedrick

Much has been made of the supposed immunity of some varieties of plums to black-knot. In the vicinity of this Station, where the disease is always present and often rampant, the differences in immunity are not very marked in varieties of the same species. The Trifloras are less attacked, however, than any other group of plums, and the Insititias rank next in immunity. No variety of the Domesticas has yet proved to be free from the disease but strong claims are made that Middleburg and Palatine are relatively free.

Next in order of seriousness among the diseases which attack cultivated plums is the brown-rot[157] (Sclerotinia fructigena (Persoon) Schroeter) known also very commonly as the ripe-rot and sometimes as peach-blight. The disease is most conspicuous on the ripe fruits of the various drupes and is popularly supposed to be confined to the fruits alone. Such is not the case, for it also attacks, and very vigorously oftentimes, the flowers and shoots. The presence of the disease on the fruits is known by a dark discoloration of the skin which is afterward partly or wholly covered by pustule-like aggregations of grayish spores. The decayed fruits may fall to the ground, or as is more usual in the case of plums, they hang to the tree and as the juice evaporates become shriveled mummies, each mummy being a storehouse of the fungus from which infection spreads the following season. The twigs, flowers and leaves are known to be suffering from inroads of the parasite when they are blackened as if nipped by frost. In warm, damp weather the rot spreads with great rapidity and fruits touching in clusters or in boxes stored for shipping are well placed to spread the epidemic. Destruction of the mummy-like fruits and all other sources of infection, and spraying with bordeaux mixture are now practiced as preventives, but so far as the crop is concerned with but indifferent success. A better remedy than we now have is eagerly looked for by growers of fruits.

The hosts of this fungus show varying degrees of susceptibility to it, the peach and the sweet cherries being more subject to it than plums. Similarly, among plums some species and varieties are more susceptible than others. Thus the Trifloras and Americanas, the latter especially in the South, are injured more by the brown-rot than other species. The idiosyncrasies of varieties in this regard are best shown in the discussions of the individual sorts.

Several interesting and sometimes destructive diseases of plums are caused by various species of the fungal genus Exoascus.[158] The most common of these, and the most striking and destructive, is plum-pockets (Exoascus pruni Fuckel), which causes prominent deformities of the fruit. These give the disease the common name or less frequently “bladders” and “curl.” The fungus attacks the developing fruits at an early stage of their growth and causes the production of a spongy mass in the fleshy tissue which greatly enlarges and distorts the plum. The stone in a diseased plum is but rudimentary or very often not at all developed. Less prominently but quite as frequently, the leaves are attacked, showing as they unfold more or less red or yellow with a very decided curling and arching of the leaf-blade. The disease usually spreads from the leaves to the shoots, the infected shoots with their rosettes of mal-formed leaves giving the tree a most unsightly appearance. Prevention at present consists of removing the diseased parts and spraying with bordeaux mixture when the buds begin to swell. Munsoniana and Hortulana plums seem to be most susceptible to this disease. Atkinson[159] has described several species of Exoascus on the different species of wild plums, some of which are liable to be found on the cultivated varieties of the native plums. They are all very similar to Exoascus pruni, differing chiefly, in the eyes of the layman, in forming smaller pockets. Sturgis[160] records an attack of one of the leaf-curl fungi, distinct from the plum-pockets fungus, on varieties of Triflora in Connecticut, which seemed to him to be of scientific and economic importance.

The leaves of the different species of cultivated plums are attacked by several fungi which produce diseased spots on the foliage, which for most part drop out, causing a shot-hole effect. These diseases pass under such descriptive names as “shot-hole fungus,” “leaf-spot,” and “leaf-blight.” The fungus probably responsible for most of this trouble is best known as the shot-hole fungus[161] (Cylindrosporium padi Karsten). The Domestica and Triflora varieties are very susceptible to this fungus, which, on the foliage of the first, causes spots for most part, while on the latter the spots on the leaves are nearly always followed by holes. Varieties of the native species, especially those of Americana and Nigra, are relatively free from this disease. Another of these shot-hole fungi is Cercospora circumscissa Saccardo[162], much less common than the former, but still to be considered and especially on the foliage of Americana. All of these diseases of the foliage are prevented to some degree by the proper use of bordeaux mixture, which, on the Triflora plums at least, must be used with great care to avoid injury. Cultivation has a salutary effect as it destroys the diseased leaves which harbor the fungi.

Another disease of plum foliage, occurring rarely on the fruit, is the plum-leaf rust[163] (Puccinia pruni-spinosae Persoon) which produces so considerable a number of spore cases on the underside of the leaves as to give the foliage a brownish cast and to cause defoliation in severe infections. The fungus is most apparent in the fall and most troublesome in warm, moist climates. Bordeaux is used as a preventive.

Stewart and Rolfs have shown that trunks and branches of plums affected by sunscald in New York are almost invariably infested by a fungus[164] (Valsa leucostoma Persoon) which in the Old World is known as the “die back” of the peach. The disease manifests itself on plums chiefly by affected areas much depressed at the boundary between the living and the dead bark, these areas usually, not always, having connection with sunscald injuries on the trunk. The disease is accompanied by more or less gumming.

In common with nearly all rosaceous plants, in nearly all countries, the plum is sometimes seriously injured by the powdery mildew[165] (Podosphaera oxyacanthae DeBary). The affected leaves have a grayish appearance caused by the parts of the fungus which project beyond the leaf tissue; when badly diseased the leaves are more or less arched and curled. Mildew is seldom prevalent enough on plums to require treatment.

The crown gall,[166] (Bacterium tumefaciens Smith and Townsend) is a parasite on all of the fruits of the order Rosaceae and is especially common on nursery stock, attacking plums in many soils but rarely, however, to the great injury of the plant. These galls are perennial structures of very varying duration. They are to be found on the roots, usually at the collar of the plant, and vary from the size of a pea to that of a man’s fist, forming at maturity, rough, knotty, dark-colored masses. Means of prevention or cure are not established though all agree that soils may be inoculated with the disease from infected stock; hence the necessity of discarding diseased trees at transplanting time.

Smith found in Michigan and Clinton in Connecticut a disease of the fruit called bacterial black spot[167] (Pseudomonas pruni Smith) of the same generic origin as the crown gall but widely different in nature. The writers and the growers who found the infected fruit, saw the disease only on the Triflora plums. It attacks the green fruits which show conspicuous, black-purple, sunken spots sometimes as large as half an inch in diameter. The injuries are usually isolated and quite superficial but nevertheless, spoil the fruit.