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| Azalea gall caused by Exobasidium |
Many times I receive phone calls from the public concerning
plants in the landscape or vegetable garden becoming sickly, showing symptoms
of some kind. The first feeling I get when receiving these phone calls is a
feeling of excitement, as if I am a detective assigned to a case.
First things first, gather and organize the clues. The clues
are the in the form of symptoms, signs and environmental condition in which the
victim, I mean plant, is found. The symptoms are defined as any abnormality in “normal”
growth of the plant. Signs are evidence of a pathogenic agent on the
symptomatic plant; a few examples are mycelium (white fuzzy strings) and spores
for fungi, and ooze for bacteria. The signs in combination with symptoms and
pattern of symptomology on individual plants and/or groups of plants can paint
a beautiful story for the detective or plant diagnostician.
Plant pathologists utilize the disease triangle as their hierarchical
concept to determine the disease. Within the disease triangle, there are three
parameters that work in a matrix to determine level of disease; they are
susceptible host, pathogen, and environment. The level of each parameter will
work in synchrony to determine the output, which is disease progress and
severity.
I will survey some common problems I have seen this Spring:
Fire blight of pear and relatives:
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| Shepherd's crook symptom on flowering pear. |
Fire blight is a disease common in Southern Louisiana caused
by the bacterium Erwinia amylovora.
Over 130 species of plants including: pear, apple, quince, hawthorne,
contoneaster and mountain ash, are susceptible to this pathogen. Symptoms are
observed as Shepherd’s crook necrotic foliage, in which the leaves appeared
scorched and remain on the petiole or stem. This disease has been observed on Pyrus spp. including ornamental and
fruit species. This disease is favored by warm, wet weather and spread by
pollinators and wind driven rain. Improper fertilization, either too much or
too little also favors this disease. Managing fire blight in the landscape is
mostly management of fertilizer, planting resistant cultivars, pruning infected
wood and lastly, chemical control.
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| Fire blighted pear tree |
When pruning out diseased tissues, cuts
should be made 12-14” down from the symptomatic tissues in the growing season
and 8-10” down from symptomatic tissue in the dormant season. Pruning shears should be sterilized with
isopropanol between cuts to prevent the spread of the bacteria between cuts.
Copper fungicides, such as Kocide 2000, can be sprayed following the label
recommendations for the particular plant; keeping in mind copper can be
phytotoxic to some plants. You should always read the label before spraying
fungicides.
Powdery Mildew on rose
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| Powdery mildew on rose leaf |
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| Conidia in white mat on rose flower |
Powdery mildew is a disease caused by a fungus and can
affect many plants including roses, crapemyrtles, dogwoods and many cucurbits. The
disease is usually favored by warm dry weather by day, and cool damp weather by
night, usually falling in or around the spring and fall for southern Louisiana.
The disease is observed with new growth being distorted and dwarfed often
covered in a mat of white “fuzzy stuff”, and old growth and flower buds
appearing twisted and distorted also covered in white fuzzy growth. The white
fuzzy material are the asexual spores of the fungus called conidia. The conidia
are produced in chains and are abundant. This is how the pathogen spreads so
quickly under favorable environments. Proper pruning to thin out the canopy,
proper fertilization, and avoiding overhead irrigation can help prevent this
disease. This disease can be managed with the following sprays: 3% baking soda and horticulture oil solution,
noting the temperature limitations on oils, chlorothalonil, and copper
sulfates. You should always read the label before spraying fungicides.
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| Conidia on rose leaf magnified at 4.5x with dissecting microsope |
Daylilly Rust
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| streaking yellow symptom on laylilly |
Daylilly rust is a disease caused by the fungus
Puccinia hemerocallidis. The most
obvious symptoms of this disease are yellow streaks turning brown with age on the upperside of the
leaf coupled with orange pustules of spores found on the underside of the leaf
observed as yellow to orange raised areas at individual lesions. The disease was first identified in the United States in 2000, and quickly spread to 30 states by 2001. The disease is spread by wind carried spores that are developed on lesions. The rusts are a particularly interesting group of diseases in that many of the species of fungi that cause rusts require two completely distinctive hosts to complete their life cycle. In the case of daylily rust, daylilies are the primary host, where species of
Patrinia are the secondary
host where sexual reproduction occurs and new genetics are generated. The best
preventative method of management is by using resistant cultivars, and avoiding
highly susceptible cultivars. Lists of resistant and susceptible cultivars can
be found at this website:
http://plantclinic.cornell.edu/factsheets/daylilyrust.pdf. Another management strategy to limit the
severity of this disease is to remove infected, dead tissues in the winter and
burning or sending them to a landfill. In addition, daylily growers should
avoid close proximity plantings of
Patrinia
spp. and Daylillies so the rust cannot complete its life cycle. Preventative
fungicide sprays can help reduce the devastation of this rust disease. A few
sprays available to the home owner are under the chemical names neem oil,
potassium bicarbonate, sulfur, or Bacillus subtilis (Serenade, Plant Guardian).
You should always read the label before spraying fungicides.
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| Rust pustules on underside of daylily leaf. |
Phytophthora foot rot of citrus
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| One sided blight of citrus typical of foot (root) rot |
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| foot rot at graft union on navel orange tree |
Phytophthora foot rot is the most frequently encountered
disease on trunks of citrus trees in Louisiana. The disease is caused by a
soilborne fungus, and wet conditions during the spring favor fruit rot development.
Initial symptoms include water soaking of the bark that appears as a dark spot
on the trunk. At first, the bark appears firm, but with age it becomes cracked
an may shred as it dries. This disease is managed indirectly by managing good
drainage of the planted area. Citrus trees should be planted in a well-drained
area, and weeds and mulch should be avoided under the tree. Planting in raised
beds with adequate drainage and pruning low hanging branches to allow air
movement around the tree are the best management strategies for the homeowner.
When the days get increasingly warmer and wetter, a surprise
explosion of mushrooms begin to appear in our lawns and along road sides.
Typically, these mushrooms grow in a ring that we call a “fairy ring”. There
are many fairy ring stories in European folklore. Most center on the belief
that the rings were places where elves or fairies would dance. The legends warn
against humans disrupting or joining the dance, lest they be punished. In
France, they believed that giant bug-eyed toads would guard the mushrooms and
humans who disrespected the ring would be cursed. Since, these tales have been
disproven and we know now that the rings of mushrooms are the sexual stage of
the fungus that spends most of its life underground, we can disregard getting
cursed and can easily remove the unsightly mushrooms before they open up into a
parasol. The mushrooms produces spores in gills after the cap has fully
expanded. These spores are deposited and theoretically can produce more fairy
rings in the future given the right environmental conditions. The rings will
increasingly get larger as years progress, and growth and mushroom production
is dependent on optimum moisture and temperature, which are the two major
factors that induce fructification. .
What will peppers will be in the garden?
