There is the possibility that injury in advance of bacterial invasion is due in part to a deleterious byproduct produced from the host tissue. Not uncommonly the affected bark tissue of plums contains large numbers of crystals that have the refractive index of calcium oxalate. A number of bacteria have been reported to produce oxalic acid from the fermentation of glucose. That this acid may accumulate in sufficient amounts to cause injury to the host tissue is questionable because it is readily combined with calcium, forming calcium oxalate, which, because of its insolubility, should be relatively noninjurious to the tree tissues.

No marked effect of soil fertility on the disease has been noted. In tests I conducted, peach trees that received an application of ammonium sulfate in spring survived an attack of bacterial canker somewhat better but were no less susceptible to subsequent infection than nonfertilized trees. According to results of trials conducted in England, neither the nitrogen, potassium, nor phosphorus content of the soil affects the susceptibility of plum trees, although applications of lime apparently increased susceptibility.

WE HAVE CONSIDERED the general subject of susceptibility to bacterial canker among species and varieties of Prunus and included in the discussion both the fruit-producing and the rootstock types. We shall now consider what happens when the susceptibility characteristics of both types occur in the same tree. First, however, we should note that most of the trees planted in California are produced by inserting a bud from the desired fruit-producing sort into the base of the trunk of the appropriate rootstock. Later the trunk of the rootstock is cut off just above the bud, which, upon growing, produces essentially all of the above-ground part of the tree. Presumably, therefore, the effect of bacterial canker on such trees depends largely on the susceptibility of the overstock, for, as we noted earlier, this disease is almost always confined to the above-ground parts. In fact, to our knowledge, bacterial canker never attacks the roots directly and rarely does the canker extend from the trunk into the roots.

Though much of our information on root involvement came from trees propagated on the usual rootstock, some of it came from seedlings trees on their own roots. If any difference existed between bud-propagated and seedling trees with respect to root infection it was so slight as to be overlooked. Trees propagated by that method, therefore, should rarely be injured or killed by bacterial canker in the root system. Hence, the statement that the effect of the disease in Sikh trees depends largely on the susceptibility of the overstock seems well founded.

ANOTHER method of propagation is that of grafting the trunk of the rootstock several feet above ground with scions from the desired commercial sort. Cherry trees are commonly propagated in that way in England. For trees so produced, the effect of the disease is determined both by the susceptibility of the understock and the susceptibility of the overstock. Inasmuch as infection of the trunk may result in death of the entire tree, the susceptibility characteristic of the understock is of primary importance in trees propagated in this manner.

Attempts to provide the tree with a resistant trunk have been tried by various persons. M. C. Goldsworthy and R. E. Smith of the California Agricultural Experiment Station found that trees consisting of commercial plum varieties grafted high on myrobalan and Marianna rootstocks survived somewhat longer than trees produced by budding this rootstock near the ground. In a severe outbreak of bacterial canker in 1929, however, many such trees were lost. Later experience with the Marianna selection 2624 shows it to be highly susceptible in the disease. The myrobalan seems much more resistant and one selection, the Myrobalan B, produced in England, is said to afford a satisfactory trunk for susceptible varieties of plum.

Likewise, investigators in England regard the Kentish Bush plum to be a superior understock on which to work the Giant Prune and Victoria plum. Several selections of mazzard, particularly the one designated F 12/1, developed at the East Malling Research Station, are said to afford a resistant trunk for sweet cherry.

We have considerable evidence that the rootstock may influence the susceptibility of the scion. According to investigators at the East Malling Station, England, certain mazzard selections apparently induce susceptibility to canker and leaf spotting in the Bigarreau de Schrencken cherry growing on them, while others tend to increase its relative resistance.

Some years ago I observed a similar phenomenon on Santa Rosa plums. An outbreak of green shoot infection by Pseudomonas syringae was much less severe on trees produced by grafting plum scions onto peach trunks 4 or 5 feet from the ground than on trees produced by budding the plum into peach rootstock near the ground. More recently, another investigator in California found evidence that apricots on peach root are less susceptible to bacterial canker than apricots on myrobalan root, whereas apricots on apricot root are intermediate in susceptibility. He also reported that plums top-worked on peach trunks (in these cases the Salwey and Lovell peaches) were more resistant than plums that were top-worked on myrobalan and Marianna trunks.

Needless to say, many questions regarding the effect of rootstock on susceptibility of the scion remain unanswered. One in particular is this: Why, on the same rootstock, do some Plum varieties prove less susceptible when top-worked than when budded near the ground? The explanation of this anomalous situation is possibly found in the fact that trees of these Plum varieties, when budded near the ground, produce roots from above the bud union and in so doing gradually become established on their roots. Thereafter, presumably, the susceptibility of the above-ground parts of such trees is no longer modified by the original rootstock.

REMOVAL OF branches infected with bacterial canker has a place in control procedure. If such branches remain in the tree, the bacteria in them will be spread to nearby trees by rain and the cankers will continue to spread into more and more of the tree. Whether the removal of individual cankers is feasible depends upon the number of cankers and their extent. It may be more practical to remove the entire branch than to spend time on the removal of diseased tissue. In any event, such surgical work should be done in the summer when activity of the disease is at its minimum.

We have attempted, without success, to treat affected branches with chemicals for the purpose of killing the bacteria inside the tissue. Zinc chloride, a material formerly employed in treating fire blight cankers, proved to be injurious. Such chemicals as copper nitrate, phenolic compounds, and sodium arsenite were ineffective.

Workers in England have paid considerable attention to control of the disease by spraying. The program adopted for that climate consists of an application of bordeaux, 10-15-100, in autumn for protection of the branches and an application of bordeaux, 6-9 100, in spring for protection of shoots and leaves. Such a program is said to reduce the incidence of bacterial canker materially in the susceptible Bigarreau de Schrencken variety of cherry. A similar program has been recommended in Australia.

For some years, cherry growers report benefits from an application of bordeaux, 16-16 100, after leaf fall in autumn.

E. E. WILSON is professor of plant pathology in the University of California at Davis where he has been engaged in studying fruit diseases since 1929. His studies of bacterial canker extended over a 9-year period. Dr. Wilson is also on the staff of the California Agricultural Experiment Station.