January 22, 2004
Master Gardener Training:Plant Pathology Lecture Outline
I. Definition of Plant disease: disturbance or irritation over time that
damages a plant or
prevents normal plant development, reducing economic or
aesthetic value
II. Symptoms
A. Infection (biotic pathogen) or Exposure (abiotic problem)
B. Latent period ºEarly symptomsºAdvanced symptoms
C. Secondary invaders of senescent or dead tissue
III. Collecting samples for diagnosis
A. Collect a representative sample,
including multiple small plants or plant
parts without symptoms, with early symptoms, and with advanced
symptoms. One leaf, leaflet, flower, or fruit is often not sufficient.
B. Observe the
site carefully and ask questions for clues (see handout
#21, "Prompts Master Gardener Volunteers can use when diagnosing
plants)
C. Avoid drying
the sample. Avoid wetness by loosely wrapping sample in
dry paper towels or newspaper, then placing in a plastic bag. Avoid heat
by
transporting in insulted container (Styrofoam, etc.). For some samples, it may
be necessary to contact the laboratory or specialist by telephone or e-mail
ahead of time for other instructions.
IV. Disease triangle
A. Virulent pathogen [some pathogens require a vector (carrier)]
B. Conducive environment for infection and for pathogen growth/reproduction/spread
C. Susceptible plant (host)
V. Disease control requires changing one or more parts of the disease triangle
A. Pathogen
1. Exclusion [No trespassing!]
a. Disease-free plants and seeds
b. Pathogen-free soil, potting mix, topsoil
2. Sanitation, competition, parasitism [Let them starve!]
a. Rotation [avoid supplemental plant hosts of pathogen]
b. Fallow [no plants, including weeds]
c. Roguing
whole plants, pruning/ hand picking diseased parts
d. Crop termination
e. Cultivation
f. Composting [some pathogens may survive composting]
g. Clean tools, stakes, containers, clothes, gloves, boots, etc.
h. Do not increase insect vector numbers with use of unnecessary
insecticides
that harm beneficial insects
i.
Biological control (e.g., Trichoderma, Bacillus subtilis)
3. Eradication [Now we’re getting serious!]
a. Solarization (only effective in top few inches of soil)
b. Heat and/or steam (practical for small quantities of soil or compost)
c. Gas (metam sodium, chloropicrin, methyl iodide, etc.)
4. Therapy
a. Non-systemic pesticides
(protectants)
b. Systemic pesticides
c.
Induced resistance [e.g. Actigard
TM (acibenzolar), Messenger
TM (hairpin)]
B. Environment
1. Optimum plant-growing-conditions are usually different from optimum
disease-conditions
2. Irrigation
a. Placement
b. Frequency
c. Quantity
3. Site
a. Air movement
b. Sun, shade, reflected light
c. Temperature (air, soil)
4. Soil
a. Drainage (surface and internal)
b. Preparation (cultivation, beds, etc.)
c. Organic matter
d. Mulch
5. Nutrition
6. Spacing
7. Planting date
C. Plant
1. Propagate existing resistant variety or native plant
2. Resistant variety developed from existing natural variation by controlled
matings (crossing), evaluating
thousands of the offspring, selecting a new
variety
[may take >10-15 years]
3. Resistant variety developed introducing foreign genes
[transgenics]
VI. Pathogen types
A. Virus
(plural: viruses)
1. Transmission
a. Vegetative (cuttings, tubers, rhizomes, runners)
b. Seed (through female flower parts and/or through pollen)
c. Vector (aphid, thrips, leafhopper, beetle, whitefly, mite,
nematode, fungus)
d. Mechanical (hands, clothing, equipment)
2. RNA or DNA, single or double stranded
Procaryote: any self-contained cell or organism
that lacks internal
unit membranes
B. Bacteria (singular: bacterium)
1. Bacteria with capability of epiphyte-phase on above-ground and below-
ground
plant parts; or saprophyte in soil, in
debris
a. Leaf spots, blights, cankers
b. Vascular wilts
2. Bacteria infecting vascular tissue with help of insect vectors
a. Xylem-limited bacteria vectored by sharpshooter/leafhopper
insects (examples: Pierce's disease of grape;
bacterial leaf scorch of
oleander, red oak, sycamore,
cedar elm, etc.)
b. Vascular wilt bacteria vectored by beetles
3. Bacteria that
insert tumor-inducing plasmids into exposed
plant cells)
[Agrobacterium tumefaciens is often used on dicot plants as
a vector for
foreign genes in
transgenic variety development]
C. Phytoplasmas
(singular: phytoplasma) [procaryote]
1. Insect vectors (example: "aster yellows")
2. Vegetative transmission (a desirable example: dwarfing and branching
of
poinsettias)
Eucaryote: any cell or organism that possesses a clearly defined nucleus
D. Stramenopiles (singular: stramenopile);
also known as water mold
"fungi" and oomycetes; produce motile
zoospores and longsurviving
oospores; related to brown algae
1. Downy mildew
2. White "rust" (Albugo species)
3. Phytophthora (example: late blight of potato, Phytophthora root
and crown rot, Phytophthora aerial blight of periwinkle [vincal])
4. Pythium
(examples: Pythium damping off, crown rot & root rot)
E. Algae (singular: alga) [only in wet, humid East Texas]
1. Algal leaf spot of southern magnolia
2. Algal cane lesions on blackberry
F. Plants that are parasites
1. Mistletoe and dwarf mistletoe
2. Dodder ("love" vine)
3. Witchweed
4. Broomrape
[Ball moss
and Spanish moss (tillandsia spp.) are nonpathogenic epiphytes,
as are certain ferns and mosses. Lichens (fungus/alga colonies) are also
common epiphytes on plants.]
G. Fungi (the true Fungi)
1. Club fungi (Basidiomycetes)
a. Rusts, smuts, wood rotting fungi, certain leaf spots
2. Sack fungi (Ascomycetes)
a. Powdery mildew, most leaf spots, certain root rots, oak wilt
and other vascular wilts
3. Imperfect fungi (sexual stage has never been found, or is rare)
a. Cotton root rot, aflatoxin in corn (Aspergillus flavus), numerous
leaf spots, root rots, Fusarium wilt and other vascular wilts,
anthracnose
H. Animals [nematodes (singular: nematode)]
1. Root knot and cyst (very low damage threshold; few + big problem)
2. Lesion, sting, ring, etc. (high damage threshold; many = perhaps problem)
I. Abiotic (non-parasitic) problems
1. Nutrient deficiency [iron (Fe), zinc (Zn), nitrogen (N), phosphorus (P)]
a. Unavailable [certain minor elements may be present at high levels in
soil but not readily available to plant roots (example: Fe and Zn become
chemically bound in soil at high pH, especially at high P levels)]
b. Elements may be at low levels in soil
c. N leaches through soil with excess water. Also, N moves within the
plant if needed at the growing point
2. Mineral toxicity
a. Fluorine (Fl) or boron (B) may be present in ground water in western
or northwestern Texas; also in certain fertilizers
b.Aluminum (Al) or manganese (Mn) are "released" in very acid
(very low pH) soils
3. Salts
a. Irrigation water
b. Soil
c. Fertilizer
d. High pan evaporation climate (hot, sunny, low humidity)
e. Composed sludge/manure ("dillo-dirt")
4. Temperature extremes (high; low; sudden drastic changes)
5. Light extremes (quality, intensity, day length)
6. Soil moisture extremes
7. Pesticide (toxicity, contamination, drift)