Phytophthora attacks a lot of trees that Tree Croppers are interested in. It can quickly kill a tree. It is a soil-borne fungus that needs moisture and ideal temperatures to get established. It has different types of structure and spores; some that die quite quickly in dry soil, but others that survive as resting bodies and begin active growth again when the ideal moisture and temperatures return. Therefore, infected soil is a latent problem waiting for a susceptible tree to come along. Once it is established, it is impossible to eradicate completely.What can be done to avoid it?The first thing is site selection if growing trees that are susceptible. The disease thrives in wet boggy soil with poor drainage. That is why it is especially problematical in wet warm climates. A free draining soil is that first sensible precaution.Secondly, improving drainage of the soil by sub-soiling, mole drains, deep ripping and laying pipes to carry excess water away will help. Thirdly, some species have so-called resistant varieties. These don’t want abusing by expecting miracles, but will help the overall approach. In other words, use all the other improvements possible together. Fourthly, there are treatments such as Foli-R-Fos (ethyl phosphite), used as soil, leaf and injection applications, that give good protection for some crops. Heavy liming (especially gypsum) seems to be beneficial. So is heavy mulching, probably by improving soil aeration, through worm and other microbe action and decaying activity. The last suggestion is for planting on a "Bern" or rise, so that excess water drains off. The most important thing is good management after planting to manage soil water. This includes the way you irrigate. There are 16 species of phytophthora known so far; 8 are named, and 8 not named yet. But in New Zealand, two are usually mentioned; P cactorum and P cinnamomi. However, what you are usually up against is a company/combination of species, including Pythium and Fusarium. This makes resistance difficult. I believe P cactorum is the major one in NZ. Susceptibility of different crops and rootstocks Species known to be susceptible are:Apple; the knowledge of the relative resistance of rootstocks is unsure because of the difference in various reports. However, mm106 is susceptible, and mm793, m9 and m26 have resistance, and should be used on Moutere clays.Pear can be attacked, but are generally quite tolerant. Stonefruit (summer fruit) peaches, apricots, plums, almonds are all highly susceptible, with mazzard and morello rootstocks more resistant. Walnut information varies considerably, but J hindsii and Paradox all have some tolerance. Chestnuts are often susceptible, with C mollisima and C crenata very susceptible, and C sativa relatively resistant. We are currently testing some possibly-resistant crenata. Causal OrganismsAlthough it is generally believed that phytophthora are involved in most crown rot situations, it would be unwise to say that other fungi can’t produce similar symptoms. However, the frequency of finding phytophthora with crown rot symptoms in a variety of crops strongly suggests that it is the primary cause of the disease.The true situation is rather confused. Life Cycle of P cactorumAerial sporongiophores bearing a succession of sporongia are produced on the infected host tissue when the atmosphere is moist. Under favourable conditions the sporongium produce zoospores which emerge at the apex of the sporongium. Biflagellate zoospores are liberated when the wall breaks. This may occur within six or seven hours. Under unfavourable conditions the sporongium may develop into a structure called conidium, that when released, if kept dry will remain dormant for months, and germinate when moisture is present again. There is another resting structure called chlamydiophore. There is also a sexual reproduction on the hyphal branches resulting in zoospores that under go a period of dormancy like seeds. During this period of several months, they must be in a moist environment to survive.Disease DevelopmentPhytophthora can live in the soil without a host plant for extended periods, probably obtaining nutrients from dead plant material. Nurserys' healed-in trees have been a source of infection.The disease is more likely to occur when the soil remains wet for extended periods. Growth can cease when the relative humidity falls below 90%, and below 60% the active fungus soon dies. The most favourable temperature seems to very between 10°C and 27°C. Development of the disease would also advance in wet conditions by injury to the tree roots. We should all know that oxygen aerated soil is needed for the health of the roots. Young non-bearing trees seem somewhat less susceptible to infection than older bearing trees. ConclusionGood healthy well-aerated soil with good drainage should mean you have little problem with phytophthora. In wet boggy conditions you are asking for trouble.Kia ora.
http://www.treecrops.org.nz/knowl/addgen/phytoph.html |
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