Mir’s Top Ten Horticulture Problems
Number 10. Tree Decline and Dieback: Common in trees that have had no care for several years. There
is a reduction in growth, chlorotic leaves, twig dieback, premature fall coloration, defoliation, cankers,
borers, fungus and mushrooms, along with root decay. Solution: Deep root feed with liquid fertilizer
plus added zinc and iron. Tender loving attention will go far in helping the tree to come back from the
edge of dying. A healthy tree will withstand the assault from fungus and insects.
Number 9. Slime Flux: Often called wet wood is a bacterial disease found in many types of trees. It can
be seen as a white to gray foam or brown ooze bubbling out of the bark and on very hot days it will
steam or smoke. The substance oozing is highly acidic and will discolor the bark and kill the grass or
plants living under the tree. Wounds caused by improper pruning are often the leading cause. Solution:
Prevention is essential. Pruning correctly and not damaging the tree with the lawn mower will help
prevent the wounds needed to introduce the bacteria. There is no cure for the heartwood infection that
is most common in elm and poplar. However the cambial infections which builds up pressure inside the
tree can be corrected if the infected limbs and tissue is promptly cut away. Sometimes drilling a hole can
relieve the pressure too. When removing infected areas, disinfect the tools after each cut. Cut around
the infected area until you come to healthy tissue and shape the wound into a large oval, clean with
rubbing alcohol or a 10% bleach solution and watch for repeated infections.
Number 8. Iron Chlorosis: This is the most common micronutrient problem for ornamentals, shrubs,
vines and trees in Idaho. The leaves are yellow, light green or white with dark green veins. This is caused
by the plants inability to extract iron from the Idaho arid soil. Other factors contribute such as excessive
moisture, high concentrations of phosphorous, copper, manganese and zinc in the soil as well as high
temperatures. Solution: First, it is highly preferable to select plants that are iron efficient and you may
need to replace those plants that are suffering. Correcting iron availability in soil can be expensive.
Secondly, controlling soil moisture with good water management will make a big difference. Too much
or too little water can change the chemistry of your soil and prevent the plants from grabbing the
necessary nutrients needed.
Number 7. Blossom End Rot: Symptoms of discoloration are found on the blossom end or opposite the
stem. These legions will enlarge, turn black or brown and become sunken into the fruit. This is caused by
a deficiency in calcium. Solution: Cultural practices help the plants uptake of calcium. Good water
management will reduce water stress and using mulches will help keep moisture more uniform.
Number 6. Fruit Tree Insects: Many insects infest fruit trees in Idaho; such as codling moth, Cherry fruit
fly, leafminers, and borers. Borers can be found in the stems, trunk crowns, or root systems. Leaf insects
reduce photosynthesis and cause fruit damage. Temperatures above 60 generally end the hibernation
of many insects and their eggs. Once warmer weather has set in, wasps, flies, moths, crawlers and other
types of insects emerge to mate and lay eggs. Solution: The first thing that all gardeners can do to
reduce the number of pests is to clean up your areas in the spring and as fruits fall. Insects overwinter in
debris, dead trees and fallen fruits. Remove dead trees that will host numerous pests. Pest control
methods are very important to controlling the pest populations. Backyard growers must take efforts to
control in order to minimize these pests from affecting the commercial trees. Although life cycles of
insects vary; cultural, biological and chemical methods can be used at periodic times of the season to
reduce the effects of these insects. Protecting our beneficial insects is also important. These beneficials
will sometimes provide enough late season predatory control to limit the need for pesticides. And the
last thing you can do to protect your trees is a late dormant oil treatment in the fall. Oil is cheap and
very effective in smothering overwintering unhatched eggs.
Number 5. Aphids and Spider Mites: These insects are the very small, soft-bodied bugs that are
frequently found in large numbers sucking sap from roots, stems, and leaves. The aphids produce a
sticky honeydew that attract other insects including ants who have been known to herd and protect the
aphids. These insects will also vector various plant diseases causing more damage. Spider Mites use
webbing that traps dust and makes trees and shrubs look dirty. Large populations of mites cause leaves
to bronze, dry up and fall prematurely. Solution: First, increase the biological insects such as ladybugs,
lacewing, parasitic wasps and predatory mites. Secondly, disrupt their habitat with regular strong water
sprays that will dislodge the pests, killing some and disorienting many. Third, keep control of weeds that
give mites and aphids a place to overwinter. Fourth, and finally, organic applications of insecticidal soaps
or oil is highly effective. Preventative applications of dormant oil each fall will prevent many of these
insects from hatching.
Number 4. Powdery Mildew: This is one of the most wide-spread diseases in Idaho. The plants will be
covered in a white to grayish powder-like growth. Infected leaves can become distorted, turn yellow
with small patches of green and fall off prematurely. This is a fungi that feeds on the upper epidermal
cells of the plant and overwinters in plant debris and is easily spread with water and splashing raindrops.
Solution: Many varieties of plants have been bred to resist powdery mildew so selecting these varieties
is helpful. Nonresistant plants shouldn’t be planted in low lying areas with high shade. Removing all
debris and burning infected plant material in the fall is important. Increase air circulation by pruning. If a
certain type of plant continues to get the disease, remove completely and choose a different plant.
Another organic solution is to mix 2.5 tablespoons of horticulture oil, 1 tablespoon of baking soda with 1
gallon of water. This has been found to be very effective in killing off the fungus and protecting the
beneficial insects in your garden.
Number 3. Other Fungus: Anthracnose or leap spot is a fungus disease reported quite often in Idaho. It
seems more common when there is a wet spring. Leaf symptoms show tan spots that develop purple
rims. Leaves may also have necrotic veins and leaf margins, and large necrotic blotches. In some cases
shot holes appear. Solution: Cultural & preventative controls work to prevent the problem. Trees and
plants receiving good care will be better able to withstand anthracnose. Maintain by watering well
during periods of drought. Mulching can help to reduce watering needs as well as protect trunks from
mechanical injury. Avoid overhead watering to minimize the chance for leaf infections. Effective control
is possible only if the disease is detected before extensive dieback occurs. Always rake up fallen leaves
and dispose of all fallen twigs to reduce spreading. Remove succulent branches as they form to prevent
trunk canker formation. Avoid high nitrogen fertilizers which stimulate succulent branching. Trees with
poor vigor will be bolstered by deep root feeding with a balanced liquid fertilizer in early spring.
Chemical or organic controls have to be used when fungus has taken its hold. Apply a general chemical
garden fungicide---please read all directions carefully and used as directed. Or you may use other
organic alternatives to chemicals such as: Insecticidal soap or a Baking Soda mixture: Actually this may
come closer to killing off fungus than most fungicide. It’s also nontoxic. Mix 2 teaspoons per gallon of
water, adding a few drops of dish soap to help the solution stick to the plants.
Number 2. Fire Blight: This is one of the most destructive plant diseases found in Idaho. It affects plant
genus’ from the rose family. (apple, pear, rose, spirea, hawthorn etc.) When damage is typically seen
first on twigs and young shoots, then it is already too late. You will see areas (cankers)that look burnt
with bacteria oozing out. Plants are most susceptible in the spring when blossoms are open or when
damaged by weather or equipment. Solution: Proper fertilization is a must. Bacteria overwinters on
cankers and as the temperatures rise, the cankers ooze the bacteria which can be taken with the wind or
rain to other plants. Cut out cankers and infected tissue. Dispose of infected tissues by burning.
Chemicals are not effective after infection occurs.
Number 1. Unhealthy lifeless soil: The number one horticulture problem seen by most people is the
one most overlooked. Matter-of- fact, it is estimated that 90% of the problems that gardeners
experience can be traced back to the condition of the soil. Your soil may be dusty, gray with not a worm
in sight. It has been compacted with little room for any roots to move. What you must understand is
that soil is where it all begins. Solution: There is just one thing that will cure all ills of soil. Adding good
quality compost (organic matter) will turn any soil type into the rich, airy living soil that plants crave. It
will be full of oxygen, teaming with good bacteria and micro-organisms; loose and spongy for the roots
to push down in. Plants can’t help but grow and in turn, attract the good insects to keep the bad pests
at bay. It achieves the perfect balance for your space on this earth.
If you find your plants or trees experiencing any of these problems or you’re not sure what the problem
is, bring a sample to the Extension Office at 535 E. Jackson for expert recommendations. You can also
call us at 587-2136 ext. 509 or email us a picture of your ailing plant to elmore@uidaho.edu.