Fire blight is becoming increasingly problematic on Washington apples. It is particularly troublesome on M9 and M26. The M26 rootstock is extremely suscpetible to blight. Erwinina amylovora, the fire blight bacterium, can enter the tree through blossoms or wounds, travel to the rootstock, and kill it very quickly. The only cure for rootstock blight is preventation. Resistant rootstocks, good orchard sanitation, and bactericide sprays are the two main weapons against this disease. Because E. amylovora has developed widespread resistance to streptomycin (Agri-Strep), it is virtually useless for blight control in Washington. Thanks to the issuance of Section 18's, growers have been recently been premitted to use terramcyin (Mycoshield).
Collar rot is perhaps the largest killer of apple trees in Washington state, yet it receives relatively little attention. Unlike fire blight, collar rot is a slow moving disease kills a few trees every year. However, over a period of years it can drastically reduce the productivity of an infested orchard by weakening and killing trees.
A thorough inspection of the upper main root system is often necessary to diagnose this disease. Trees with collar rot show a pronounced lack of vigor, restricted terminal growth, and discolored leaves. A reddish bronze/purple leaf discoloration is frequently the first visible symptom of collar rot. Initially, leaves of affected hosts are light green in summer and reddish bronze in early autumn. Fruit on infected trees may be small and color prematurely. Diseased root systems usually reveal light tan to dark brown discoloration of the inner bark, depending upon the age of infection. Collar rot results from the interaction of a susceptible host, a pathogen (Phytophthora species), and a conducive environment. These factors will be considered separately. Phytophthora cactorum is the fungus most often associated with collar rot of apple. It survives in soil in the form of a thick walled spore (oospore). These spores are capable of surviving extended periods of time in soil. Under moist conditions they germinate forming another type of structure called a sporangium. A sporangium is a minute reproductive structure produced by the fungus in response to free water. Sporangia germinate in the presence of free water releasing numerous one-celled motile spores called zospores. Zoospores swim through water, contact roots of susceptible hosts, germinate, and infect. P. cactorum and other Phytopthora species are present in most of our irrigation canals, where the biology of them is poorly understood.
Several species of Phytophthora can cause collar rot of pome and stone fruits: P. cactorum, P. cambivora, P. cryptogea, P. drechsleri, P. megasperma, and P. citricola. The latter three are usually confined to stone fruits. Most rootstock collar rot resistance ratings have been developed by evaulation for resistance to P. cactorum. Our experience with rootstocks and collar rot in Washington reveals that M.26, M.104, and M.106 are highly suceptible, M.2, M.7, and M. 111 are moderately resistanst, and M.4 and M.9 are highly resisant. However, even M.9 occasionally gets collar rot. Why? Probably because it has recently been show (by Browne and Mircetich of the University of California at Davis) that in addition to P. cactorum, P. cryptogea and P. cambivora can cause collar rot of apple, and that the resistance of a specific rootstock is dependent upon which species of Phytophthora is attacking the rootstock. For example, M.9 is highly resisant to P. cactorum but highly susceptible to P. cambivora; MM 106 is extemely susceptible to P. cactorum but highly resistant to P. cryptogea.
Why the concern about all of these different species of Phytophthora? We have all detected all of the ones pathogenic to apple in several of our irrigation canals. We know very little about their biology in the canals, except that their occurrence is appearance in a specific canals varies greatly from year to year. A grower must assume that his irrigation water is contaminated by one or more species of Phytophhora. We have also baited several unknown Phytophthora species from Wenatchee area canals that are pathogenic to apple and cherry roots.
What type of environment promotes collar rot? It can be summed up in three words: water, water, water. Flooded or poorly drained soils promote collar rot because they provide the environment necessary for the production of the swimming infective spores. Flooded soils may also stress and predispose the trees. Research has also shown that apple trees are most susceptible to infection during the bloom period. It is therefore necessary to avoid flooding during this time.
An integrated approach is the key to long term control of this disease. It involves site selection, rootstock choice, and chemical control. Avoid heavy soils and poorly drained sites. If collar rot problems are suspected in certain location but it is still desirable to plant apples, avoid MM 104, MM 106, M.26, and Mark rootstocks. Ridomil (metalaxyl) and Aliette (fosetyl-A) are labeled for collar rot control on bearing/nonbearing and nonbearing trees, respectively. They should be used in a protectant fashion because once trees are infected, they cannot usually be cured. It is imperative that Ridomil be applied in early spring to protect trees during the bloom period when they are most susceptible.
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Tree Fruit Research & Extension Center, Washington State University,1100 N Western Ave., Wenatchee, WA, 98801 USA
Wenatchee WA, 31 October 1995