10 Common Fruit Tree Diseases in the Portland Area
Growing fruit in the Pacific Northwest can be a very fun and rewarding experience. Unfortunately, the climate of the Willamette Valley can be a challenging factor for home and commercial orchard production because regular precipitation and dew in spring and summer allows fungal and bacterial diseases to thrive.
Areas with low spring and summer rainfall and abundant sunshine, such as North Central Washington, experience far less pressure from these diseases than we do here in Portland. This climate factor is a large part of the environmental variable in what's known as the disease triangle.
The disease triangle is a relationship between a susceptible host, a virulent pathogen, and a favorable environment. All three factors are necessary for any disease to occur. While pesticide-based agriculture focuses on controlling diseases after they occur, ecological disease management focuses on avoiding the conditions that predispose plants to disease.
In an effort to bring more understanding to the diseases most commonly encountered in Portland area orchards, here is an introduction to our top 9 bacterial and fungal diseases and the methods of cultural and biological control used to avoid and address them.
1. Fire Blight
2. Pseudomonas bacterial canker
3. Apple Scab & Pear Scab (see scab blog)
4. Perennial Canker/Bull's Eye Rot/Anthracnose
5. Powdery Mildew
6. Speck Rot & Sphaeropsis Rot
7. Brown Rot
8. Silver Leaf
9. Shot Hole Blight
10. Leaf Curl
Fire Blight is a bacterial disease affecting apples and pears. It is caused by the bacterium, Erwinia amylovora, which grows between 50° and 100°F and most rapidly between 75° and 90°F. The bacteria overwinter in blight cankers (diseased bark tissue) infected the previous year. Leading up to and during bloom, bacteria will ooze from the cankers and is spread by rain as well as insects which are drawn to the ooze and spread the pathogen to flower stigmas as they feed on the flower nectar. The bacteria multiply during the 4-8 days the flower is open and if a fruitlet is successfully colonized, the infection can spread to the cambium layer between bark and wood, killing host tissue. One active overwintering canker can produce enough bacteria to infect a significant portion of flowers in a one acre area so it is important to cut out cankers in the dormant season and remove damaged wood from the orchard. Working together with neighbors to minimize blight infections can lead to a healthier, more productive season for all.
An integrated approach to blight prevention includes selecting resistant rootstock and cultivars, selecting higher elevation planting sites for less resistant cultivars, training trees to limit humidity and maximize air flow and sunlight, organic insect and microbial control, bloom thinning, and soil and foliar nutrient management. By focusing pruning efforts in the summer and early fall, and reducing dormant pruning, it is possible to reduce excessive spring shoot growth thereby reducing shoot blight. Especially with summer pruning, it is important to sanitize tools frequently. In summer, cutting out blight at least 12" below the edge of the visible canker can reduce the risk of additional damage. In extreme cases, a late dormant organic copper and oil spray can reduce bacteria levels and a fall oil application can reduce overwintering infection. Cultivars with shorter bloom periods generally fare better than cultivars with longer and multiple bloom periods.
Fire Blight Resistant* Apple Rootstock- Bernali, Budagovsky 119, Budagovsky 490, Geneva Series, Malling 7, Vineland: 1,2,5,6,7
Fire Blight Resistant* Apple Cultivars- Akane, Arkansas Black, Britemac, Carroll, Classic Delicious, Cox's Orange Pippin, Scarlet Gala, Stark Splendor, Swiss Gormet, Turley, Delicious, Early Red One McIntosh, Gold Rush, Gold Spur, Harolsen, Honeycrisp, Jamba, James Grieve, Jonafree, Keepsake, Kidd's Orange Red, Liberty, Laurared, Lustre Elstar, Lysgolden, Macfree, Macspur, Empire, Melba, Melrose, Mor Spur Mac, Nova Easygro, Nured Delicious, Nured Winesap, Ozark Gold, Perfect Spur Criterion, Pioneer Mac, Prima, Red Max, Red Winesap, Regent, Remo, Rubinola, Stamared, Stark Bounty, Priscilla, Reanda, Red Delicious, Red Delicious,
Starkspur, Starking, Starkrimson, Scarlet Spur, Sturdeespur, Topspur, Dixi, Dana, Ace, Red Chief, Viking, Wellington, William's Pride, William's Red
Fire Blight Resistant* Pear Rootstock- Pyrus betulaefolia ‘Old Home x Farmingdale’, Pyrus calleryana, Pyrus communis ‘Old Home’, Pyrus communis ‘Old Home X Farmingdale’
Fire Blight Resistant* Pear Cultivars- Ayers, Beurre Bosc, Bradford, Carrick, Harrow Delight, Harrow Sweet, Honeysweet, Kieffer, LeConte, Magness, Maxine, Moon Glow, Old Home, Tyson, Waite, Warren
Bacterial Canker is a bacterial disease affecting cherries. It is caused by the bacterium, Pseudomonas syringae, and favors cool, wet weather. The bacteria overwinter in cankers and host tissue. Dark cankers on trunks and branches expand in spring. The main symptom is a sunken, dark canker on the trunk and main scaffold branches. Infected tissues sometimes produce gum and the cankers can also girdle branches, causing dieback above the lesion. Infected areas result in dead buds, and yellow, collapsed and dead leaves by late summer. Bacterial canker is spread by wind, rain, insects, pruning tools, or infected grafting and planting stock. The disease may be systemic and may not show visible symptoms.
Cultural management practices for bacterial canker include avoiding injury to trees, cutting out infected tissue during dry weather, sanitizing tools frequently, and removing severely infected trees. Some orchardists cut out and cauterize cankers in early spring before bloom. Additionally, the bacterium colonizes many weeds and grasses so keeping grasses in check and considering clover and vetch as these cover crops support lower bacterial canker populations. In young trees the infection often occurs in areas of tissue damage from winter freezes and extreme frost, insect damage, pruning cuts, or leaf scars. In mature trees infection often occurs following heading cuts. Bacterial canker is reduced when trunks are painted white to reduce sun-scald injury. Trees planted in well-drained soils with proper irrigation and nutrition are far less susceptible to infection than stressed out trees. If possible, all cherry pruning should be addressed in summer when conditions are dry. Copper sprays have been shown to increase bacterial canker infections.
Bacterial Canker Resistant* Rootstock- Colt
Bacterial Canker Resistant* Cultivars- Rainier, Regina, Sandra Rose
Apple Anthracnose, Bull's Eye Rot, and Perennial Canker are fungal diseases caused by fungi in the genus, Neofabraea. Host plants include crabapples, most pome and stone fruit, shadbush, hawthorn, and mountain ash. The fungus causes cankers on most commonly on younger, smaller branches and bull's eye rot on fruit. Rain in fall spreads fungal spores to maturing fruit and young branches. In fall, a small, circular, red-brown spot on the bark is the first sign of infection. New canker development and extension appears in spring, followed by a clear crack around the canker with tissue inside the crack shrinking, shriveling, dying, peeling off and leaving an area of exposed dead tissue with 'fiddle-string' appearance. In summer, fungus spores mature and are scattered by rain and wind by early fall. The bull's eye rot phase of the disease appears as brown circular spots on fruit in storage, eventually with fruiting fungal bodies in concentric circle's. Perennial Canker i is common in areas with hot and dry summers and severe winters east of the Cascades. Perennial cankers grow in concentric circles as their development is renewed annually by small galls caused by the woolly apple aphid around the callus edges of existing cankers. These galls serve as new sites for infection and eventually constrict the sap conducting tissue leading to severe damage or death of the tree.
Cultural control focuses mainly on aggressive scouting and removal of infected areas to remove the inoculum source and pruning out and burning affected branches. Severely infected trees should be removed and burned.
Powdery Mildew is a fungal disease that affects cherries, apples, pears, peaches, plums, nectarines, caneberries, grapes, strawberries, and many other ornamental and vegetable plants. Unlike many other diseases, powdery mildew doesn't require moist conditions to grow and is limited by wet springs. The fungus overwinters in buds of deciduous plants and on leaves that remain through winter on other plants. Powdery mildew is recognized easily by the white to gray mycelium and fungal spores that form on leaves, flowers, fruit and shoots.
Cultural Control- Provide good air circulation and sunlight by pruning excess foliage. Prune out infected buds in the dormant season and severely infested shoots as they appear in early spring.
Powdery Mildew Resistant* Cultivars- Apple: Red Delicious and Stayman Winesap. Peach: Freestone varieties are less susceptible to the disease.
Powdery Mildew Susceptible Cultivars- Apple: Gravenstein, Jonathan, Rome Beauty, Yellow Newton. Cherry: Bing, Black Tartarian, Rainier. Peach: Elegant Lady, Fairtime, Fay Elberta, Summerset. Plum: Black Beaut, Gaviota, Kelsey, Wickson.
Speck Rot and Sphaeropsis Rot are post harvest fruit rots caused by the fungi, Phacidiopycnis washingtonensisand Sphaeropsis pyriputrescens respectively. Speck rot affects apple and the Manchurian crabapple variety which is highly susceptible to the disease and has acted as an infective host to commercial orchards because it is often planted among market apple trees as a pollenizer. Sphaeropsis rot affects apples and pears and in particular the D'Anjou pear and the Red Delicous, Golden Delicious, Fuji, and Granny Smith apples. The Manchurian crabapple is highly susceptible to sphaeropsis rot as well as speck rot. Both diseases survive on dead tissue and cause twig dieback and cankers and spread via tiny black fruiting bodies which produce spores that are spread with rain. Fruit decay caused by sphaeropsis rot has a distinct bandage-like odor in fruit flesh and speck rot is distinguished by dots around either the calyx end or stem end of an apple.
Cultural Control for both of these fungal rots includes pruning to allow adequate sunlight and air flow and removal of dead and diseased twigs, branches and fruit from the orchard.
Brown Rot blossom and twig blight is caused by the fungi Monilinia fructicola and sometimes Monilinia laxa and occurs in various pome and stone fruits. The disease overwinters in mummified fruit and then release spores into the air when trees are in bloom. When infected, young blossom spurs and surrounding leaves collapse, gum exudes from the base of flowers and cankers with tan centers and dark margins form on twigs. Infection of mature fruits occur in injured tissues and is evident when the fruit is ripe at higher temperatures.
Cultural Control-The disease is managed by similar fungal control methods of open pruning and removal of infected fruit and shoots. The infection can be checked by prompt storage and rapid cooling to low temperatures, especially before it is well established in fruits.
Silver Leaf is a fungal disease that affects cherries, apples, pears, peaches, plums, nectarines, blueberries, alder, willow, and poplar. It is caused by the fungus, Chondrostereum purpureum, which attacks roots, trunks, branches and shoots. The fungus is a saprophyte, meaning it lives on dead tissue. It can become an active parasite if there is an entry point to living tissue via a pruning cut, insect damage, mechanical injury, or winter injury. As the name suggests, leaves that are dark green under healthy conditions turn silver on one or two small branches, eventually turning to an ashy grey with curled edges and quickly infecting larger branches which will eventually die. Once a branch is dead, the fruiting body of the fungus exudes from the dead tissue and forms a bracket fungus which is usually salmon pink to purple underneath, tiered and 1 to 3 inches wide.
Cultural Control-Burn prunings, avoid injury to trees, prune properly and in dry conditions. There has been some biological control with the fungus Trichoderma viride.
Shothole or Coryneum Blight is caused by the fungus Thyrostroma carpophilum and affects cherry, peach, and apricot trees. The fungus overwinters on infected twigs and buds and spores are spread by water. The clearest symptoms are holes in the leaves of infected trees up to 1/4 inch in diameter. The lesions start as dark brown, red or purple spots, sometimes with a light green to yellow halo. When temperatures rise in spring, the spots die and drop, leaving a perforated or "shothole' appearance on the leaves. Cultural control includes pruning to increase air flow and light penetration and avoiding overhead irrigation. If cankers are present on twigs or bark, prune out the infected areas.
Leaf Curl: Taphrina deformans, is a fungal disease commonly found on stone fruit in and around Portland. If you have a peach tree you have likely seen this disease. The fungus overwinters on infected leaf and twig tissue, and like scab spore release happens in spring during wet periods where the temperature hangs out between 50-70 degrees. Leaf curl can infect every new leaf emerging in the spring, causing leaves to wither and die within a few weeks. This has the impact of stressing the tree to the point of effecting fruit set and production. The tree will often drop all infected leaves by mid summer and the resulting new growth will look fine. However, the damage has been done and overtime the vigor and health of these trees will decline. Your best bet for growing peaches, nectarines, and apricots in and around Portland is to plant a resistant variety. Look for Puget Gold Apricot, Frost Peach, Indian Free stone Peach, Muir Peach and Kreibich Nectarine for highest resistance. Avoid Redskin Peach and it's relatives as they are highly susceptible.
*Resistant ≠ Immune
What to spray on your fruit trees to help prevent diseases:
Organic Sprays we recommend using with proper discretion according to labels and safety warnings: Liqui-Cop, Wettable Sulfur, Neem oil, Potassium bicarbonate, and Bacillus Subtillis. All are generally allowed in organic food production but should only be used when particular care is given to pollinator activity levels at the time of application, dilution rates, and the scope of the orchard production. It is rare that a home-scale fruit tree necessitates the use of organic chemical control. It is always better to manage long-term orchard health and stimulate beneficial microorganisms and predatory insects.
The holistic spray we generally use is based on Michael Phillips recipe from the Holistic Orchard Network:
Makes 1 gallon
one ounce of pure neem (100% the stuff that is solid at room temp, not the processed stuff)
a teaspoon of natural soap (to emulsify the neem oil)
3 oz of liquid fish
1 tablespoon of Molasses
1 tablespoon of kelp extract
During spring weather where we see temps from 50-70 degrees and lots of precipitation we add: 4-6 oz, biological fungicide (bacillis subtilis)