Bacterial Canker of Stone Fruits

E. E. Wilson.

Bacterial canker, bacterial gummosis, and bacterial sour sap are names given to a disease of stone-fruit trees.

The disease is found in the eastern and midwestern parts of the United States, but it is of major consequence only in the States bordering the Pacific Ocean. Even there it varies in frequency and severity. It is said to be rare in the cherry districts of Oregon east of the Cascade Mountains but is often destructive west of the Cascades. In the Sierra Nevada foothills of central California it is severe on plums but much less severe in the central valleys.

Its names denote its main features. An abnormal oozing of gum from the affected branch suggests the term bacterial gummosis. Gum flow, although common in cherries and apricots, is by no means a constant feature of the disease on them or other stone-fruit trees. Some of the most severe cases in the Sierra Nevada foothills are accompanied by little or no gumming; the exudate, when present, is a watery material that flows from moist, sour-smelling, discolored areas of the bark.

Bacterial canker, on the other hand, is a more appropriate term because the diseased areas on branches are always necrotic lesions or cankers.

The disease affects many parts of the tree, but the most common and most destructive phase is that on the trunks, limbs, and branches. There the pathogen enters the bark and makes circular to elongated, brown, water-soaked or gum-soaked lesions in the bark and outermost sapwood. Branches girdled by the canker may fail to grow in the spring. If they produce leaves and grow for a period, they die the first warm days of summer. The affected bark tissue is brown, gum-impregnated or water-soaked, and sour smelling.

Bacterial canker may kill a tree by girdling the trunk, but it seldom extends below ground. The root systems of trees thus killed not uncommonly develop a number of new shoots within the year.

The second most destructive phase is the blighting of the dormant buds and the subsequent killing of the twigs that produce the buds. Cherry and apricot are particularly susceptible to infection of dormant or partly opened buds. On them the bacteria kill the buds and produce a small canker on the twig at the base of the bud. On some plums, however, extensive invasion of the twigs commonly follows infection.

At times, in California, bacterial canker attacks blossoms of cherry and plum. The flowers die soon after they open. Loss of crop through blossom infection is infrequent, but it may be heavy when it does occur.

In England the disease is said to attack plum and cherry leaves extensively, causing much defoliation, which lowers the vigor of the tree. In California the usual dry weather of late spring and summer minimizes such infection. Occasionally a few trees in an orchard may lose their leaves, but no extensive or recurring outbreaks of leaf infection are experienced. Leaf spotting, when it occurs, is often associated with severe infections of branches. Bacteria washed from such cankers by rain presumably cause infection of the surrounding leaves.

The fruit of the apricot and cherry trees sometimes develop sunken, black lesions bordered by water-soaked margins. The lesions later become even more sunken as the fruit enlarges. Lesions also occur on the fruit stems.

Sudden witherings of the new leafy shoots of Santa Rosa, Wickson, and Duarte plum varieties have been seen several times in California. That is a common and destructive manifestation of bacterial canker in England, because, once established in green shoots, the disease invades the older twigs. Under our conditions, seldom more than a few terminal shoots are affected and the disease rarely extends from those parts into the older twigs. Moreover, with us the bacteria apparently do not survive in the summer in affected shoots.

THE CORRECT DESIGNATION for the organism causing bacterial canker of stone-fruit trees is now considered to be Pseudomonas syringae. For a number of years the pathogen was known either as Pseudomonas cerasi, Bacterium cerasi, or Phytomonas cerasi, according to the system of classification followed at the time. The changes in the names typify the complications that arise in classification when an organism has a wide range of hosts and has been studied at various times by various persons. At one time or another this organism has been given no less than 12 species names, each one designating it as a pathogen of some particular host or group of hosts. Pseudomonas. syringae was the first species to receive proof of pathogenicity and an adequate description when, in 1902, C. J. J. Van Hall, of the University of Amsterdam, reported that it produced a blight of lilac.

In the following list, the binomials now considered to be synonymous with Pseudomonas syringae and other pertinent data are given: Bacillus spongiosus, by R. Aderhold and H. Ruhland on cherry in Germany (1907); Pseudomonas cerasi, by F. L. Griffin on cherry in Oregon (1911) ; Bacterium citriputeale, by C. O. Smith on citrus in California (1913); Bacterium citrarefaciens, by H. A. Lee on citrus in California (1917); Pseudomonas hibisci, by K. Nakata and S. Takimoto, on the hibiscus in Japan (1923); Pseudomonas vignae, by M. W. Gardner and J. B. Kendrick on cowpea in Indiana (1923); Bacterium viridifaciens by W. B. Tisdale and M. M. Williamson on lima bean in Wisconsin (1923); Bacterium trifoliorum, by L. R. Jones et al. on clover in eastern United States (1923); Bacterium holci, by J. B. Kendrick on Holcus species and other hosts in Iowa (1926); Pseudomonas prunicola, by H. Wormald on cherry, plum, and other hosts in England (1930); Phytomonas utiformica, by F. M. Clara on pear in New York (1932).

H. Wormald, in England in 1931, described still another species (Pseudomonas mors-prunorum) that causes a canker condition of stone-fruit trees in England. P. mors-prunorum retains its species name even though it is admittedly very similar, both culturally and physiologically, to P. syringae (P. prunicola). One gets the impression that P. mors-prunorum most frequently is the cause of severe infection of branches, whereas P. syringae most frequently is the cause of infection of leaves and green shoots, but no great consistency is exhibited in these features. No purpose is served by attempting here to distinguish between the symptoms of the two.

PSEUDOMONAS SYRINGAE is encountered in many parts of the world on lilac, most species of stone fruit, pear, apple, citrus, avocado, clover, cowpea, lima bean, common bean, rose, sorghum and related grasses, corn, pearl-millet, foxtail, hibiscus, and forsythia. Pseudomonas syringae var. capsici has been reported on pepper from Italy.

Of the species of Prunus cultivated for their fruit, the apricot (P. armeniaca) is probably the most susceptible. Next are the plum (P. domestics and P. salicina) and the sweet cherry (P. avium). The peach (P. persica) and nectarine (P. persica var. nectarine) are on the whole less susceptible than the sweet cherry. The almond (P. amygdalus) is only occasionally damaged. The sour cherry (P. cerasus) and the duke cherries (hybrids between P. cerasus and P. avium) are rarely found in California, so little is known about their susceptibility under our conditions. Sour cherries are highly resistant to the disease in Oregon, and the duke cherries are rarely affected.

Of the species of plums used as rootstocks, the Marianna (hybrid between P. cerasifera and a native plum) is prone to infection. The myrobalan (P. cerasifera) is affected somewhat, but is much more resistant than Marianna. From England have come reports of two resistant rootstocks of plum, the Myrobalan B and Purple Pershore.

Susceptibility among the mazzard cherry (Prunus avium) rootstocks varies greatly, because seedlings of both the wild and cultivated sweet cherries are grown for the purpose. One, the vegetatively produced F 12/1. developed at the East Malling Research Station in England, is said to exhibit a high degree of resistance.

The mahaleb cherry (P. mahaleb), used quite widely as a rootstock for sweet cherries in California, seems to be resistant to bacterial canker.

Only one selection of P. cerasus is grown as a rootstock in California. It is the Stockton morello, which is commonly obtained by planting the root suckers from trees on this root. It appears to be relatively resistant to bacterial canker.

Formerly all rootstocks of peach were seedlings obtained from commercial varieties; Salwey and Lovell were most popular. In one district where bacterial canker is prevalent those varieties appear to be little affected. Two other rootstocks, the Shalil and Stribling's S 37, have come into use because of their resistance to root knot nematode. Their susceptibility to bacterial canker is not known.

Records of tree losses indicate that the common varieties of sweet cherry are susceptible in the following descending order: Lambert, Napoleon (Royal Ann), Bing, Chapman, Lewelling (Black Republican), and Black Tartarian. Lambert is listed as less susceptible than Napoleon or Bing in Oregon. Lambert is listed as very resistant in England.

The plums grown in California belong to two species, P. domestica and P. salicina. A few varieties, however, apparently are hybrids between P. salicina and other species. The more important varieties exhibit tolerance to the disease in the following increasing order (the letters in parentheses indicate whether the varietal characteristic is predominantly that of P. salicina (S) or that of P. domestica (D), no attempt being made to distinguish the hybrids): Duarte (S), President (D), (D), Climax (S), Giant (S):

Grand Duke (D), Tragedy (D), Santa Rosa (S), Wickson (S), Burbank (S), Formosa (S), Gaviota (S), Beauty (S), Kelsey (S).

Peach varieties such as the Elberta and its variants, Fay Elberta and Early Elberta, the now rarely planted Phillips Cling, the Halford No. 2, and J. H. Hale not infrequently are affected severely by bacterial canker. Lovell and Early Crawford are less affected by the disease.

Among apricots, the Blenheim, Royal, and Tilton varieties are most commonly planted in Califolifornia, and all are highly susceptible.

On almond, the disease has been observed on Nonpareil, Ne Plus Ultra, and Texas.