Phytophthora root, collar, and crown rot: Phytophthora root, crown, and collar rots are common and destructive diseases of fruit trees throughout the world. In eastern North America, apple, cherry, peach, and apricot trees are most commonly attacked, whereas pear and plum trees appear to be relatively resistant. True collar rot (infection of only the scion portion of the trunk near the graft union) is relatively rare in this region, although the term often is applied to diseased root and/or crown portions of the rootstock, which are much more common. Trees declining from Phytophthora root and crown rots are frequently diagnosed as suffering from"wet feet" (mistakenly assumed to be root asphyxiation or "drowning"), and sometimes are confused with those suffering from winter injury.

Management: Management of Phytophthora root and crown rots is best accomplished using an integrated program of cultural practices and, sometimes, chemical control. Soils that are excessively slow to drain or subject to periodic waterlogging should be avoided. Marginal sites should be modified as necessary to provide the additional drainage recommended for growing tree fruit crops, e.g., install drain tiles, create diversion ditches, rip underlying pan layers, etc. Planting trees on ridges or berms will raise much of the crown area above the zone of significant zoospore activity and provides an important margin of safety, especially in a wet year. Where irrigation is practiced, avoid excessive rates of application and arrange drip emitters to avoid puddling around the trunk.

Tree species and rootstocks should be selected to match the soil and drainage characteristics of an orchard. However, rankings of resistance and susceptibility to Phytophthora root and crown rots must be recognized as generalizations only, due to the array of Phytophthora species that potentially can be involved. Pears are the fruit tree with the greatest resistance to these diseases, and are the most likely to remain healthy in a relatively wet site. Apple rootstocks vary widely in susceptibility, but generally are more susceptible than pears and more resistant than stone fruit rootstocks other than plums. Among apple rootstocks, seedling is relatively resistant, as are M.9, M.2, and M.4; M.7 (and M.7a), M. 26, and MM. 111 are moderately susceptible; MM.106 and MM.104 are highly susceptible. The new CG (Cornell Geneva) rootstock series has been bred to show resistance to some Phytophthora species under greenhouse conditions, but there is insufficient experience to determine whether this will hold under field conditions (note that MM.106 also appeared resistant in initial greenhouse trials). Among stone fruits, plums are relatively resistant, whereas the remainder are susceptible to very susceptible. Mahaleb is the most susceptible cherry rootstock, whereas Mazzard, Morello, and Colt are somewhat more resistant and would be recommended on the heavier cherry soils. Some of the newer clonal cherry rootstocks may have an additional measure of resistance, but these have not been sufficiently evaluated in the field to determine. Seedling peach and apricot rootstocks are very susceptible, although the range of suitable soils may be expanded if these fruits can be grown without other problems on plum-type rootstocks.

Preplant soil fumigation should not be relied upon to control Phytophthora root and crown rots, since the fumigant never completely eradicates existing inoculum from the soil and Phytophthora spp. are easily reintroduced. Some fungicides have activity against Phytophthora and a narrow range of related organisms, and can be effective when used preventively in combination with the cultural practices described above. Because Phytophthora disease occur sporadically, it is seldom economical to treat entire orchards on a regular preventative basis. Rather, it is best to target such applications to sections of the orchard and seasons that are most conducive to disease development (e.g., wet spots or orchard sections with previous Phytophthora problems, excessively wet spring and/or fall seasons). Also note that these fungicides seldom are effective in reviving trees once the crown has become infected and moderate symptoms of decline have appeared. Check current labels and recommendations for approved materials and timings.

Fosetyl-al (Aliette), applied to less than 1% of the acres at an average rate of 2.0 lbs. a.i. per acre. It is used as a preplant treatment for nonbearing trees only. It can also be applied as a foliar spray.

Metalaxyl (Ridomil), applied to less than 1% of the acres at an averagte rate of 1.25 lbs. per acre. The application rate varies with the method of application and the size of the trees. Applications are made in early spring (before budbreak) and fall (after harvest).