A disease can be described as any change from the normal condition of plants, which detract from their appearance, or make them less useful or valuable. Here we will cover biotic diseases caused by bacteria, fungi and viruses, specifically affecting fruit trees or their fruit.
Types of Tree Fruit Diseases
Bacteria are microscopic, single celled organisms that reproduce by simple division, rapidly growing the population during ideal weather and host conditions. The presence of water is necessary for the spread of most bacterial infections, as the bacterial cells are motile (can swim in water). Plant bacteria require an opening in plant tissue in order to infect the plant. Wounds, pruning cuts, environmental stress damage, as well as natural openings in the plant can act as entry points for pathogens. Once inside the plant, bacteria can spread throughout the plant. Examples of bacterial fruit tree diseases are fire blight, and bacterial spots and cankers.
Fungi are organisms that require a food source (plants) because they cannot make their own food. Most fungi reproduce by spores. Fungal pathogens enter plant hosts in different ways. Some are capable of penetrating plant tissue, and others require openings like stomata in leaves, lenticels in fruit, flowers, or wounds.
Fungi can overwinter in infected buds, shoots, and cankers of host plants; and on dried leaves, limbs and other debris on the orchard floor. In the spring, fungi will spread by spores with rain and wind, and infect new tissues including leaves and new shoots on trees. Examples of fungal fruit tree diseases are apple scab, powdery mildew, and brown rot. Some postharvest fungal decays may originate in the orchard, but fruit show no symptoms until undergoing cold storage. Examples of such fungal decays include: Bull’s eye rot, Sphaeropsis rot, Phacidiopycnis rot. Additional fungal infections can occur at harvest through damage on the fruit skin caused by stem punctures, limb rubs and bruising.
Viruses are submicroscopic parasitic particles that depend on their host for reproduction. Viruses enter living cells, which do not recognize the virus as an invader. The host cells use valuable energy to help the viruses replicate. They spread from host to host in different ways. Vectors can be other organisms (e.g. insects) that pick up the virus, and pass it along to healthy plants (example: mealy bugs and little cherry virus 2). Some viruses are spread in plant pollen; and others by the seeds of flowering plants infected with a virus. Viruses can also be transmitted by vegetative plant propagation (e.g., budding or grafting), through natural root grafting, and by nematodes.
One of the most important things to do to avoid virus diseases is to select virus-tested and certified virus-free planting material from a reputable commercial nursery. See the Clean Plant Center of the Northwest page for information on viruses, virus-free materials, and testing services.
The Disease Process
Plant diseases can only develop when three things occur at the same time creating the “disease triangle.” The disease causing pathogen, the right environment, and an entry way into a host plant are the three elements that all need to be present at the same time for disease to occur. Minimizing the influence of any of the three components can reduce the severity of the disease. Some of the bacterial and fungal diseases that afflict fruit trees in our area start to grow and spread in the spring when rain showers and temperatures increase, with just the right windows of time for incubation, spreading bacterial cells and fungal spores to sensitive new leaves and flowers (examples: fire blight, scab). Insect pests, and environmental stresses like wind, hail, sun, or excessive rain can provide pathogens an opportunity to infect the tree through openings in tree bark, leaf and fruit tissues.
General Control Tactics
Selection of Varieties & Rootstocks
Select disease-resistant fruit tree varieties/rootstocks for your location and growing conditions. Also, buy certified virus-free trees from reputable nurseries.
Plant protection involves treating healthy plants before they become diseased, and disease prevention is essential to preserving fruit quality. Monitor trees for signs of disease throughout the year. Integrated pest management (IPM), is a multi-tactic approach to managing, among other pests, plant diseases. Disease management techniques include cultural, biological, and chemical methods, and allowing low, non-damaging levels of a problem. Chemical or biological methods can be used to control insects that cause diseases (examples: sooty molds caused by secretions from aphids and pear psylla). Cultural sanitation methods include pruning out infected twigs and limbs, removing and destroying the debris (examples:, burning or composting) so it won’t re-inoculate the trees.
Use decision support systems and tools like the WSU-DAS Cougar Blight model for fire blight to know when the disease is very likely to occur during bloom, if fire blight is already in your orchard.
Chemical protection is one of the main control methods for bacterial and fungal diseases. Treat severe infestations during the growing season. Spray treatments are applied during the dormant season to inhibit disease colonies. Orchard sanitation, pruning and quickly destroying debris from diseased tree tissue is important to prevent the spread of the pathogens throughout the tree and to other trees in the orchard. It is recommended to remove and destroy the debris from fire blight pruning. Federal and state plant quarantine regulations require the control of tree fruit diseases for fruit destined for export to foreign countries (e.g. Manchurian crab apple fungal diseases and exports to China) See the WSU Crop Protection Guide for Tree Fruit for tree fruits in Washington here.
Always read labels, and follow the state or extension recommendations.