Recent Rains Driving Phuture Phytophthora Problems

Throughout Indiana orchards, Phytophthora root and crown rot continues to be a major cause of tree death (Fig. 1), and losses in small fruit, as well (Fig. 2). The disease is caused by the several members of the genus Phytophthora, and includes (but is by no means limited to) the following species): P. cactorum, P. citricola/plurivora, P. megaspermum, P. drechsleri, P. cambivora, and P. cryptogea. These Phytophthora species belong to a group of organisms known as the water molds. It is important to note that these aren’t true fungi like apple or peach scab, or powdery mildew. Therefore, fungicides used to control other problems in the orchard (Topsin M, Nova, Orbit, Dodine, etc.) will not control this problem.

This year may be worse than most for Phytophthora showing up in the orchard, due to a combination of freezes and some really wet weather. Take special care scouting the low-lying areas of the orchard, particularly those orchards (or blocks) with heavy, poorly drained soils.

 Symptoms

The aboveground symptoms of a Phytophthora root and crown rot are remarkably vague. Plant growth slows compared to healthy, uninfected plants (Fig. 1,2). This can be determined by examining the growth of terminal bud scars to see if a reduction in shoot growth has occurred. The first symptoms to appear in the spring are delayed bud break, followed by leaf discoloration, and twig dieback. Additional symptoms appear on the foliage, with leaves wilting, yellowing, and falling off prematurely. Margins may brown and resemble scorch. Margins of leaves may brown in the summer (termed marginal leaf burn or scorch). In severe cases, the tree leafs out, and then dies when the first bit of heat or drought hits or around harvest (Fig. 3). If the infected trees survive the growing season (many leaf out and die at the first hint of drought), they show symptoms of leaf and bark discoloration and premature leaf drop in the fall.

The most obvious symptom found on affected trees is a partial or complete girdling of the crown ortrunk (Fig. 4). The crown appears cankered, and if the bark is removed, a reddish discoloration is often seen below. Careful examination of the large roots reveals reddish brown, water-soaked areas of necrotic tissue located at the base of the root where it attaches to the rootstock. Examination of the smaller roots reveals few healthy, white roots, an absence of root hairs, or dark brown, reddish brown, soft roots. The entire underground portion of the stem is usually water-soaked and brown, and the necrotic area usually extends upward to the graft union.

 Disease cycle

Phytophthora overwinters in the soil as oospores. In the presence of water and warmer temperatures, germinating oospores develop sporangiophores. These sporangiophores produce free-living zoospores that swim to suitable hosts in the presence of free water. Free soil moisture allows sporangiophores to form within hours and motile zoospores to be released soon after, in as little as 10 to 60 minutes. Therefore, poorly drained soil or wet sites favor the disease. Zoospores infect feeder roots just behind the root cap, and are also attracted to sites of injury. Infection can result in collar rot (infection above the graft union), crown rot (infection of the lower trunk and root bases), and/or root rot (infection of the lateral and fibrous root system). Oospores can also germinate and directly invade host tissue. Any injury to the root and crown provide easy access for Phytophthora. Soil pH plays little role in this disease.

Disease management- cultural

Crown rot prevention is difficult and eradication almost impossible in low-lying, poorly drained sites. Thus, in the future and whenever possible, avoid planting orchards in heavy, clay soils that drain poorly, and thereby favor infection by Phytophthora. In less than optimal sites, choosing the right rootstock is essential to disease management. That said, it is difficult to make absolute statements regarding rootstock resistance and susceptibility because there are several different species of Phytophthora that attack apple roots, and little work has been done looking at individual Phytophthora species and rootstocks. Of the rootstocks preferred by growers, none are completely resistant to crown rot. The most resistant rootstocks are M.4 and B.9, and M.9; the most susceptible root stocks are reported to be M.7, MM.104, and MM.106, with M.2 and MM.111 being somewhat intermediate in resistance, meaning they can be infected by crown rot under favorable conditions, but may be acceptable for well-draining sandy soils.

For the home or small orchard, remove the soil from the base of the tree if the tree has not yet been completely girdled and allow to dry–Drying can stop crown rot from progressing further. Home remedies suck as packing wounds or cankers with soil, thereby keeping the wounds moist, is one of the worst things you can do in this instance!

Control weeds around the trunk of apple trees as weeds can serve as alternate hosts for Phytophthora spp. Care should be taken when using herbicides to prevent drift from damaging tree crowns and visible roots, particularly in young (less than 7 years) trees. As any injury can serve as a site of infection, care should be taken when using a weed whip or with any sucker removal. Finally, reduce nitrogen application of infected trees as excess nitrogen suppresses plant defense—and actually prevents the tree from fighting off the infection.

Disease management-Chemical

For bearing apples, apply Aliette or other phosphorous acid derivatives as a cover spray on a 30-60 day interval when conditions favor disease development. These fungicides are truly systemic (the fancypants word is ‘amphimobile’), meaning product applied to the leaves will be taken to the roots; product drenched on the roots will spread via the xylem throughout the plant. These are the only fungicides that function in this way!

Under moderate disease pressure, apply 6-8 applications at 2.5 lbs per acre on a 30 day interval, or use 5 lb per acre 2-4 times at on a 60 day interval. The phosphorous acid fungicides are quickly translocated from leaves to the root system, and they are very effective for controlling Phytophthora. Young trees at risk should be protected, as these can be quickly girdled by root rot and are of particular concern (regardless of the variety or rootstock), as are any orchards on MM.106 or M.26 rootstocks, due to their susceptibility to this disease.

Numerous phosphorous acid derivatives (ProPhyt, Rampart, Phostrol, Agrifos, etc) are available. I’ve used Aliette on both ornamental and fruit crops with good results; I’ve gotten similar results with Agrifos (on apples) and Rampart (on a lot of ornamental crops), as well.

If you have a crop and want to include Aliette or a PAD, combine it with captan since the phosphorous acid derivatives (PAD) provide no fruit rot control. As we have already entered the period where these diseases are a growing concern, application of a PAD + captan would be a good strategy to start your summer disease control program, assuming you have fruit. You could switch to captan + Topsin-M in your next spray, or just continue with the PAD + captan combination.

For nonbearing infected trees, in the fall, Ridomil can be applied as a drench around the trunk (best for preventing crown and collar rot) or as a banded treatment within the drip line (best for root rot). A Ridomil drench can be applied in the early spring before growth begins, or in the fall after harvest but before the ground freezes. Ridomil and Aliette rates can be found in the Midwest Fruit Pest Management Spray Guide.

Stone fruit are susceptible to Phytophthora, as well.  For new plantings, make the first application 2 weeks after planting. Additional applications should be made at 2–3 month intervals or to coincide with periods most favorable for root, crown, or collar rot development (like now). For established plantings, the application should be made in spring before the plants start growth. Additional applications should be made at 2–3 month intervals or to coincide with periods most favorable for root, crown, or collar rot development(now).

Note: Do not dip roots of trees in, or spray bare roots with, solutions containing Ridomil. Do not apply to stressed trees.