Identifying Pests and Diseases Overview

A good way to prevent and treat the damage on fruit trees is knowing and identifying pests and diseases. Once you can spot them, you can treat accordingly. This is a list of the most common pests and diseases with images as well as a fuller description for different pests and diseases below. If you would like to know more about organic pest control and management, then see this article.

Disease/PestSignsFruit Tree

rodents (mice & voles)
broken branches, chewed on or damaged bark and shootsall fruit
powdery mildew
powdery mildew
white powder-like moldall fruit

any of the borers
visible holes or damaged areas in tree bark that ooze gumall fruit

plum curculio
scarred, deformed, rotted fruit, crescent-shaped punctures on the skin, larvae inside the fruitall fruit

fireblight
wilted shoots, well-defined areas of burnt-looking, dead foliage or bark, sticky amber ooze.apple and pear
woolly apple aphid
woolly apple aphid
cottony masses at the base of leaves, galls on shoots, general decline in tree healthapple and pear
apple scab
apple scab
spots on leaves, brown-black spots on fruitapple
cedar apple rust
cedar apple rust
red-black spots on leaves, visible spores on undersides of leavesapple
codling moth
codling moth
puncture marks on fruit holes in fruit surrounded by waste; larvae in fruit, rotted fruitapple (and sometimes other fruit)
brown rot
brown rot
brown, rotted bloom, water-soaked, sunken lesions on twigs, brown or grey mold on fruitcherry, peach, plum, apricot
canker
canker
an area of darkened and sunken bark that expands every year, amber ooze coming from this areapeach (and sometimes other stone fruit)
peach leaf curl
peach leaf curl
malformed, puckered and bubbly leaves that turn yellow, brown, and droppeach
black knot
black knot
wart-like galls that start green and then become black over a couple of yearsplum + cherry

Identifying Pests and Diseases in Detail

Apple Scab

  • Host:
    • Apple trees
  • Description:
    • This fungal disease causes olive-green to black spots on the fruit and leaves. When the leaves are infected they become dwarfed, curled and drop prematurely. This leaf loss, especially when it happens over many seasons, can significantly weaken the tree. If the fruits get infected in the early season, they can get deformed and drop early in June. Moreover you usually can’t eat scabby fruits.
  • Causes:
    • During the winter, apple scab can survive in the dead apple leaves on the ground. Then in the spring, these fungi shoot spores into the air which the wind carries to the growing leaves, flowers, and fruit. In order to start new infections, the spores have to have a couple hours of moisture. Afterwards, the infections grow into spots which produce more spores.
  • Recommendations:
    • Firstly, you can choose scab resistant varieties such as Freedom, Liberty, Prima, Jonafree, GoldRush, and MacFree.
    • Secondly, remove fallen leaves from under the tree. If you control it in the early season it will be much easier to control in the summer.
    • Spraying is also an option.

Powdery Mildew

  • Host:
    • All fruit trees
  • Description:
    • This fungal disease shows up on the leaves of trees as white, powdery mold.
  • Causes:
    • The fungi overwinter in the dormant buds. Then during the spring, they spread onto the flowers, leaves, and fruit. This fungi doesn’t overwinter in pear tree buds. However pear trees can get the fungi from nearby apple trees. Moreover, really cold winter temperatures can kill the infected buds since they are more susceptible to winter injury than healthy buds. The infection which will cause the fruit to russet can occur from about 3 weeks before bloom to 3 weeks after bloom.
  • Recommendations:
    • Firstly, plant and prune in a way so that the trees and branches aren’t overcrowded. Then prune out the areas and branches with white fungus.
    • For pears, do not plant Anjou pears within 200 m of susceptible apple cultivars. However, Bartlett, Flemish Beauty and Winter Nelis are more resistant to powdery mildew. If you know that powdery mildew occurs in your area, then do not plant susceptible apple varieties such as Idared, Monroe, Rome Beauty, Jonathan, Paulared, Gingergold, or Cortland.

Fireblight

  • Host:
    • Apple and pear trees
    • Other plants: Serviceberry, flowering quince, hawthorn, loquat, flowering almond, plum and cherry, rose, and spirea.
  • Description:
    • The tree’s branches will not only blacken but also droop rapidly as if scorched by fire (hence the name fireblight). Additionally, the bark at the base of infected twigs will become water soaked, then dark, sunken and dry, with cracks developing at the edge of the sunken area. Other signs include wilted shoots, well-defined areas of burnt-looking, dead foliage or bark, and sticky amber ooze.
  • Causes:
    • The damage mostly happens during warm, rainy spring weather. Fireblight bacteria overwinter between live bark tissue and the tissue killed the previous season. Additionally, the bacteria can enter through openings such as flowers and wounds in the spring. Then rain, wind, and pruning tools can spread the disease to other plants. Finally, in the spring, the bacteria ooze out of the cankers and attract bees and other insects which also spread the disease.
  • Recommendations:
    • Firstly, do not grow your fruit trees on poorly drained, highly acidic, or overfertilized soils. Secondly, if you see infected twigs, cut them off in the early spring at least 8 inches below the damaged area. Additionally, if you prune in the summer, disinfect your pruning tools with denatured alcohol or a 10 percent bleach solution between each cut.
    • Some resistant pear varieties include Kieffer, Moonglow, Orient, and Seckel. In fact most other pear varieties are very susceptible to fireblight. Resistant apple varieties include Enterprise, Freedom, Liberty, Prima, Priscilla, Adams, Dolgo, Jewelberry, and Liset. Susceptible apple varieties include Idared, Jonagold, Jonathan, Lodi, Crispin (Mutsu), Greening, Paulared, Rome, Sir Prize, Spigold, Twenty Ounce, York and Gala.

Brown Rot

  • Host:
    • Peach, plum, cherry, almond, and nectarine trees
  • Description:
    • The signs of brown rot include brown, rotted bloom, water-soaked, sunken lesions on twigs, and brown or grey mold on fruits. Consequently, the infected fruits become uneatable.
  • Causes:
    • The fungus can overwinter in the infected twigs and fruits, especially if they stay on the tree or on the ground. Then, spores are released during rainy periods in the spring and summer. When the spores land on any wet surface of a stone fruit tree, they can infect the whole tree within 5 hours.
  • Recommendations:
    • Firstly, choose resistant varieties. Resistant peach varieties include Babygold No. 5, Elberta, and Glohaven.
    • Secondly, be sure to prune to allow for good air circulation. Also water the tree from below so that you don’t wet the blossoms.
    • You should also remove the diseased fruits and areas of infection as soon as they happen to reduce further damage. Additionally, remove fallen leaves.
    • You can also use a sealer to close all the cuts and wounds.
    • Lastly, you can use copper fungicides or sulfur powders weekly on the infected trees starting when the blossoms are just beginning to open and continuing throughout the growing season. Moreover, it is best to 12 hours of dry weather after spraying.

Black Knot

  • Host:
    • Plum and cherry trees
  • Description:
    • This fungal disease causes black swollen galls to form on branches and sometimes the trunks of trees. For trees that are severely affected, the leaves and shoots will wilt and die on the branches with these galls. They can start off green and turn black over the years. Additionally, these swollen galls can grow up to 8 inches long and be 2-4 times the diameter of the twig.
  • Causes:
    • Firstly, the fungus overwinters in the galls on branches and trunks.
    • During the spring when the rain comes, spores are released. Then the wing takes them to the wounded branches or young green shots where they infect.
    • As a matter of fact, this disease can grow for months without any outward signs.
    • After maturing, this fungus makes the tree grow unusually large cells which form into the galls.
  • Recommendations:
    • If you see these galls, prune the infected twigs at least 6-8 inches below the infected zone. When you wait and don’t prune, the disease will grow. However make sure to destroy the pruned branches since the diseased knots can produce and release spores for up to 4 months after removal. 

Cytospora Canker

  • Host:
    • Mainly peach, can also infect apricot, plum, nectarine, and cherry trees
  • Description:
    • From April-May, oozing, light amber to dark brown gum will appear close to the infection. Then the inner bark will start to collapse. Afterwards by the second year, this collapsed area will become an elliptically shaped canker. In the following years, the bark will be damaged, malformed, and covered with a black fungal overgrowth.
  • Causes:
    • This disease is caused by the fungus Leucostoma kunzei which is also present in healthy branches. However, the disease begins when the tree becomes stressed by insect feeding, snow or ice damage, drought or other factors.
  • Recommendations:
    • Firstly, plant cold-hardy varieties.
    • Secondly, be sure to follow proper pruning maintenance.
    • However be sure not to apply excessive nitrogen.
    • Additionally, treat and control other pests and diseases such as brown rot and peach tree borer since these will also put stress on the tree.
    • Make sure the tree is healthy so that it isn’t stressed.

Cedar Apple Rust

  • Host:
    • Apple trees
  • Description:
    • The signs of this disease include red-black spots on the leaves and visible spores on their undersides. This disease can cause damage to leaves and fruit of very susceptible apple varieties, but is only a minor problem on resistant or partially resistant trees.
  • Causes:
    • This disease requires two hosts in order to complete its life cycle, an apple tree and an eastern red cedar tree.
    • In fact, it survives the winter in galls on the cedar trees. Then with the rains in the spring hornlike extrusions emerge and release spores. In fact, with the wind, these spores can travel to nearby apple trees and cause orange spots on the leaves.
    • Then 1-2 months later, the fungi can grow to on the undersides of the leaves and on the fruit. Moreover, the fruit will be infected when it is moist and the temperature ranges between 46 degrees F and 75 degrees F.
    • Afterwards, the spores from the apple trees will be released in the summer and infect the leaves of the cedar trees.
  • Recommendations:
    • Firstly, either remove nearby eastern red cedar and juniper trees within a few hundred yards of the apple trees or do not plant the apple trees closeby.
    • You can plant disease resistant varieties such as Delicious, Liberty, Nova Easygro, Novamac, and Tydeman. However varieties that are susceptible include Golden Delicious, Jonathan, Lodi, Prima, Rome, Twenty Ounce, and York.

Apple Maggot

  • Host:
    • Apple trees
  • Description:
    • The adult flies, which are about 1/4 inch long and smaller than a common housefly, cause the fruit to be misshapen and pitted. There are also small spots on the skin where the female fly has laid her eggs. When the larvae tunnel through the flesh, the pulp also starts to rot. However you can rarely see the larvae.
  • Causes:
    • These flies emerge from the ground from mid-June to mid-August. Then they lay eggs under the skin of the apples. Even though you can seldom see these larvae, they cause the fruit to be misshapen and pitted. Afterwards when the fruit drops, these larvae become pupae and overwinter in the ground. When summertime comes, they emerge as adults and repeat the cycle.
  • Recommendations:
    • If you pick and destroy the fruit from the ground once they drop, you will destroy the potential for damage next year. However do not compost these fallen apples in your yard since this will keep the infestation close by.
    • Secondly you can protect each individual apple by enclosing them with a plastic bag. This can be easy for apples that are on reachable. However it does take time.
    • Thirdly, you can trap the female apple maggot by hanging red ball sticky traps.
    • And lastly, if you do see that the red ball sticky traps have caught some female flies, you can then use some pesticides such as esfenvalerate, carbaryl or spinosad.

Western Cherry Fruit Fly

  • Host:
    • Sweet and tart cherries
  • Description:
    • The adult flies, which are about 1/5 inch long and smaller than a common housefly, cause the fruit to be misshapen and pitted. The larvae feed on the flesh of the cherries.
  • Causes:
    • These flies emerge from the ground for about a month in early June. Then they lay eggs under the skin of the cherries. When the eggs hatch in 5-8 days, the larvae burrow towards the pit where pesticides won’t reach them. Then, when they develop further, they burrow out of the cherries and drop to the ground. When fully developed, 10 to 21 days after hatching, they bore their way out of the cherries. Afterwards when the fruit drops, these larvae become pupae and overwinter in the ground. When summertime comes, they emerge as adults and repeat the cycle.
  • Recommendations:
    • Even though these flies aren’t strongly attracted to traps, the best way to monitor these flies can be through an adhesive covered yellow panel with an ammonium-carbonate lure. However just because you don’t catch any flies doesn’t mean they aren’t present.
    • Secondly, you can use ground covers and mulches around the trees to prevent larvae from burrowing into the soil and pupating.
    • Parasitic wasps can be a biological control since they attack the larvae of the cherry fruit fly. However they aren’t very effective.
    • If you detect a threat of these flies, controlling them with an insecticide is the most effective control method. See this resource from Washington State University.


If this seems like a lot of information, don’t worry. You will be able to learn and spot these common diseases and pests over time. This is especially important since knowing and identifying pests and diseases will help you grow healthier trees. If you would like to discover our fruit trees, explore our fruit tree catalog or shop page.

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