Diseases of Tree Fruit

Black knot of plum and cherry, caused by the fungus Apiosporina morbosa, may be overlooked during the growing season when the leaves are hiding the galls, but this time of year they are hard to miss, especially when they are as abundant as they are on the tree in the photo below.


UConn Photo

This is a serious disease of these trees and can eventually kill susceptible varieties. Management options include sanitation, resistant varieties and properly timed fungicides.

Where manageable, prune out all galls during the dormant season and dispose of them off-site, burn or bury them. This is because even removed galls may still produce spores that can cause new infections. Prune  6″ below the visible edge of the gall because the fungus can be invading the wood in that area prior to gall development.

This disease can affect both orchard and ornamental varieties of plum and cherry but some of the tart cherries are less susceptible. Native wild cherries are hosts of the disease and provide a reservoir of inoculum for orchards and ornamentals. It’s helpful to remove those nearby where possible. For new plum plantings (fruiting/orchard), ‘President’ is highly resistant. Moderately resistant options include ‘Methley’, ‘Milton’, ‘Early Italian’, ‘Brodshaw’, ‘Fellenberg’, ‘Shiro’, ‘Santa Rosa’ and ‘Formosa’. ‘Shropshire’ and ‘Stanley’ are considered quite susceptible.

Here’s how disease develops: Infections occur in the spring on new growth from spores produced on the surface of 2+ year old galls. Spores are produced and spread during rainy weather and shoots must remain wet for a period of time for the spores to germinate and initiate an infection. Infections can occur at temperatures of 50°F or higher when water is present for the required period of time. Over the course of the first summer, a small greenish brown swelling develops. By the end of the second summer, the gall or knot becomes hard, rough and black. These galls begin producing spores the following spring. Galls expand in size each year until the branch is girdled (killed all the way around) and then they die. Once a twig or shoot is girdled, the portion beyond the gall can’t get any water or nutrients and dies as a result. Sometimes, larger branches and trunks can become infected, presumably through wounds.

What if you have a susceptible tree and want to prevent this disease? If you know you have a source of infection (hosts with galls nearby, either wild or on a neighboring property) and you’ve had some infections, keep up with the monitoring and pruning, fertilize and water as necessary to prevent stress, and use preventive fungicides, such as lime sulfur during dormancy (organic option) or chlorothalonil or others labeled for this disease. Other than lime sulfur, applications should be made as directed on the label beginning at bud swell and until new terminal growth ceases.

More information on this disease and its control: Black knot fact sheet

By J. Allen


If you have backyard fruit trees that you’ve nurtured along in anticipation of a delicious home-grown harvest, only to be disappointed by a lack of fruit, there are a number of possible causes. Maybe you have older trees that have been reliable bearers and they’ve suddenly decided not to produce fruit.  What could be the problem?

Photo: www.arborday.org

With new, young fruit trees, it could just be their age.  Most fruit tree nursery stock is sold when the trees are only one to two years old.  Bearing age ranges from 2 to 7 years depending on the cultivar, rootstock and tree vigor.  Dwarf trees will generally begin to bear sooner than standard size trees, with the semi-dwarfs falling in between.  A tree that is growing at a moderate rate will bear earlier than one growing either too rapidly or too slowly.  Growth rate is affected by environmental conditions, soil fertility, and moisture availability.

Plant fruit trees in a sunny location with enough space to avoid root competition with other nearby plants and trees.  Competition from weeds or grasses can be reduced using cultivation, mulch or properly labeled herbicides.  Avoid excess nitrogen fertilizer.  This stimulates vegetative growth at the expense of flower bud production.  Overfertilization is one of the most common causes of reduced flower bud production in the backyard orchard.   This is due to the application of high-nitrogen fertilizer to lawn areas around the tree.  Fertilizer recommendations for fruit trees are ¼ lb. of nitrogen per tree just after planting followed by reducing or eliminating added fertilizer until the tree begins to bear.   Once trees begin to bear, if lawn fertilizer is applied that is adequate; if not, apply 0.1 lb. nitrogen per inch of tree trunk diameter. Broadcast the fertilizer over the root zone.   To evaluate whether you should increase or decrease your fertilizer rate, note the length of new shoot growth during the previous season. The length of new growth should not exceed 18-20 inches.   Of course, a lack of nitrogen and other nutrients that reduce tree vitality will also decrease flower bud formation, fruit development and fruit quality.  A soil test can be used to obtain accurate fertilizer recommendations.

Excess pruning can delay the onset of flowering in young trees and stimulate vegetative growth at the expense of flower bud formation in bearing trees.  In young non-bearing trees, prune only as needed for developing a strong, desired framework.  In bearing trees, adjust the amount of annual pruning based on the length of terminal shoot growth as with fertilization rate.  Prune out water sprouts.

If your fruit trees have an abundance of blossoms but fail to develop fruit, the most likely causes are related to the weather and pollination.  Open blossoms can be injured by freezing temperatures.  In some cases, the blossoms will still look normal but will not be able to form fruit because of injured parts. Small backyard trees can be protected if overnight freezing temperatures are expected.  Cover the trees with plastic sheeting, old bed sheets, cheesecloth or similar materials.  The cover should reach the ground to be most effective.  Another alternative is to use sprinklers.  Turn the spray on when the temperature reaches the low 30s.  Ice will form on the tree surfaces, insulating the tissues from temperatures falling below freezing.  This occurs because as water freezes, heat energy is released.  The sprinkler MUST be kept on until the ice melts on its own or more severe injury can occur.

Photo: www.starkbros.com

The other important considerations affecting fruit production relate to pollination.  First, many tree fruits are self-unfruitful and need another cultivar as a source of pollen.  This is true (with a few exceptions) of apple, pear, sweet cherry, Japanese plum and some European plums.   Peach and apricot are self-fruitful and don’t require another pollinizer tree.   When purchasing new fruit trees, it is important to understand each cultivars requirements.  When selecting cultivars, consider the bloom period.  For example, an early blooming apple will need a pollen producer that is either an early or midseason bloomer so that the bloom periods will overlap sufficiently.   Some apple cultivars are poor pollen producers and need to be grown with two other cultivars to ensure a good pollen supply for all.  These include Baldwin, Gravenstein, Stayment, Winesap, and Rhode Island Greening.  One exception to the self-unfruitful apples is Golden Delicious, which is self-fruitful.  If you’re just interested in growing one main variety of apple, plant one pollinizing tree for every 8-9 trees.

A few tree fruits require individual male and female trees; in other words female trees have only female flowers and bear fruit while male trees only have male flowers that produce pollen.  In Connecticut, these include the hardy kiwi and persimmon.  A good rule of thumb is to have one male tree for every six females.    For a blog on growing hardy kiwi click HERE.

Okay, now we have plenty of healthy blossoms and a good source of pollen; the next ingredient is the pollinator.   The most important pollinators of fruit trees are bees.   Minor pollinators include flies, wasps, beetles, butterflies, moths and other insects.  Fruit trees that require cross pollination should be spaced in close proximity to each other without crowding to increase pollination.  The better a flower is pollinated, the more seeds the fruit will have and it will be larger and more uniform. Cold, rainy or very windy weather during bloom will reduce bee activity which will result in reduced fruit set.

Some fruit trees, including apple, have a tendency to bear a large crop one year followed by little or no fruit the next year.  The flower buds of the fruit trees are set during the summer before they open.  If there is a lot of fruit developing on the tree, there are simply fewer nutrients available for flower bud formation.   This tendency can be countered by practicing fruit thinning.  For apple and pear, thin to one fruit for every three to four spurs or 4-7 fruits per yard of branch within 30 days after bloom.  Thinning peaches and plums isn’t necessary to get a crop every year, but for larger fruit, thin to a spacing on the branch of 6-8 inches.  Sweet and sour cherries, apricots and peaches will be able to produce flower buds while carrying a heavy crop.

Pest and disease management are important in maintaining the vigor of the tree and in the development of quality fruit.  Some diseases affect the blossoms resulting in a reduction of yield while others affect the fruit as it develops.  In general, diseases of the tree may reduce its vigor, decreasing its ability to put nutritional resources into flower bud formation and/or fruit development. Insect pests can decrease fruit production because they reduce plant vigor by feeding on the leaves and reducing photosynthesis or by feeding directly on the flowers or developing fruits.

J Allen

Rust?  Fascinating?  Really?  Well, not the usual kind of rust, that unwelcome brown, crispy blistering on your beautiful old porch railing or your beloved old car or truck.  It’s the rust fungi that have lives that are truly fascinating.   Imagine spending your life in two completely different cultures.   A few people do that, but it’s pretty unusual.  It’s unusual for fungi too!  Most fungi that infect plants grow in and/or derive their nutrients from the plant host, produce spores of some type, and have various mechanisms of surviving undesirable conditions such as drought or freezing temperatures.   A number of plant pathogenic fungi can infect a wide array of different kinds of plant, but even these don’t require two completely different, unrelated plant hosts to complete their life cycle.   Some of the rust fungi do (and some don’t) and their life cycles are FASCINATING!  It makes one wonder at the turns taken in the course of evolution and why things have evolved as they have.  These fungi have the most complex life cycles in the Kingdom Fungi. 

One of the more common (and interesting) rusts in the northeast is cedar-apple rust caused by the rust fungus Gymnosporangium juniperi-virginianae.    This rust and its relatives are heteroecious, meaning that they require two unrelated hosts to complete their life cycle.  In this group, up to five different spore types may be produced.  The cedar-apple rust pathogen life cycle consists of four spore types, two on apple (Malus domestica) or crabapple (Malus sylvestris) and other related hosts,  and two on eastern red-cedar (Juniperus virginiana) or one of several other juniper species. 

Cedar-apple rust galls producing telial horns on Eastern red-cedar (May 2011).


Have you noticed showy orange structures on eastern red-cedar in the past couple of weeks during the rainy weather we’ve had?   A tree with multiple orange galls looks like it’s decorated.    If you look closely (you should!), you’ll find that the orange mass is composed of many gelatinous horns arising from a brown gall.   Each horn is covered with many teliospores (spore #1).  Teliospores have two cells each and each of these two cells germinates during wet weather to produce a club bearing four smaller spores called basidiospores (spore #2).   The basidiospores are spread by wind and driven rain to young apple leaves and sometimes fruit where new infections can begin if a film of water is present.  Apple leaves 4-8 days old are the most susceptible.   The fungus grows in the leaf tissue causing yellow to orange spots with yellow or red borders.  Pycniospores (spore #3) form in pycnia on the upper surface of the leaves 1-2 weeks after infection.   Pycniospores are exuded in a yellow liquid.  These spores are not infectious but may play a role in the production of the next spore type, the aeciospores.  Aeciospores (spore #4) are produced in aecia on the lower sides of the leaves or on infected fruit about 1-2 months after the appearance of pycnia.  In late summer aeciospores that are windblown to  young twigs of eastern red-cedar may start new infections there.   A gall will develop at the infection site through the next full growing season and in its second spring will produce telial horns, completing the life cycle of the fungus.  The life cycle  takes two years to complete and an individual gall of G. virginianae  will only produce telial horns for one season.   During that season, horns may be produced up to 8-10 times during rainy weather until the supply is exhausted. 

Telial horns of Gymnosporangium juniperi- virginianae on Eastern red-cedar.

This disease does not usually cause significant harm to its host trees except in the case of very susceptible apple cultivars.  On these hosts (apple), fruit may be disfigured and significant defoliation may occur. Resistant cultivars are the best way to minimize disease in areas where this pathogen and the alternate host occur.   For management of cedar-apple rust on susceptible apple trees, fungicides are used.

J Allen