Plant Pathology Dept.
Mimeo. Series No. X 5W-
CONTROL OF DISEASES OF FOLIAGE PLANTS
H. N. MILLER
Plant diseases must be considered major hazards in the production of
any crop. How a nursery Is managed, what the grower does, where and why
it is done must necessarily be influenced by the realization that disease
producing organisms may be ever present and that a disease outbreak can
make the difference between profit and loss.
As a grower becomes more specialized, the fewer types of plants he grows,
the less he can afford losses from disease. In the production of foliage
plants, the growing conditions ideal for the plants may also be ideal for
the growth of disease producing organisms.
Diseases caused by soil-borne organisms
Damping off, seed decay, stem rots, and root rots of nursery plants are
most commonly caused by Rhizoctonia spp. and the water molds, Pythium and
Phytophthora spp. Less important is the Southern blight fungus Sclerotium
rolfsii, and certain host specialized organisms.
Rhizoctonia and water molds
1. These organisms produce no important air-borne spores.
2. Spread is largely through splashing water or the scattering
of soil or plant fragments in which they are present and may
a. Soil is spattered by drops or a jet of water
from rain or irrigation.
b. The fungus is spread by dipping cutting in x
water or other solutions.
c. Soil on hose, tools, benches, pots and flats '.
and on workmen's hands or feet. g
d. Pots and flats are placed on infested soil.
e. Infected plant materials are planted. Ary
3. Rhizoctonia causes a decay of plant stems near the soil
surface, rather than at the root tips as with the water molds.
4. In Rhizoctonia infections coarse brown threads of the fungus
may be seen, with a hand lens, clinging to the decayed plant
parts. Soil particles hang to these threads.
5. Infection:from water molds usually start as a black, water
soaked rot of root tip, and may spread to rot all below ground
parts of the plants.
Other root-rotting fungi
1. The Southern blight fungus grows over the surface of nursery
soils rotting the stems of plants with which it comes in contact.
2. Small, seed-like bodies are formed which effectively
spread the fungus.
3. Other fungi may live in or on the soil causing specific
diseases of certain plants.
Severity of disease caused by the soil-borne organisms
1. Susceptibility of the host, which varies greatly for the
different plants, will determine the severity of
2. The quantity of the organism in the soil determines the
potential severity of the disease.
3. Soil salinity, at different concentrations may injure
the plant and increase severity of Rhizoctonia damping
4. Soft, succulent growth increases susceptibility to damping off.
5. Attacks by water molds can be reduced by maintaining the
soil as dry as will permit plant growth.
6. Soil temperatures effect the severity of disease. The
water mold usually does more damage in cool wet soils.
Control of diseases caused by soil-borne organisms
1. Soil treatment with heat.
a. Steam treatment of soil remains the best method
of disinfection for all fungi, bacteria, nematodes
and weed seed.
b. Heat soil to 180F for 30 minutes. In many
methods the temperature cannot be stopped short
of 212 F.
c. Treat soil in stationary or mobile boxes containing
a perforated pipe grid arranged in rows 9 inches
d. Soil in benches or ground beds may be steamed in
place by laying a canvas hose or perforated pipe
on top of the soil down the center of the bench
and covering the soil area with a plastic cloth.
The steam pipes or tile may be buried in the soil.
e. An inverted plan may be used over the soil for
sterilization of benches and ground beds.
f. Advantages of steam more effective, dependable
and cheaper in the long run. Steam can be used
near living plants and there are no hazards.
g. Hot water can be used for sterilizing soil. Large
amounts of water needed. Best used for sand.
2. Soil treatments with chemicals
a. On large soil areas, erradication of
pathogens by chemicals is not usually
b. Chemical treatment of soils should be done
when the soil is in good planting condition
and soil temperatures are 65 to 75 F.
c. Methyl bromide is very effective especially
for flat and bulk soil sterilization. Simple
to use and has shortest treatment and aeration
periods. To be effective against most soil
pests should be used at rate of 4 lbs./100
sq. ft. of soil surface.
d. Formaldehyde has been in longest use for soil
fumigation. Commercial formalin (37 to 40 per
cent formaldehyde) in water solution at 1 pint in
6 1/4 gals. of water is applied at rate of 1/2 gal.
per sq. ft. of soil surface.
e. Vapam at 1 to 2 quarts per 100 sq. ft. and Mylone
85 W at 3/4 lbs. per 100 sq. ft. are effective
for soil in green-house benches or ground beds.
Vapam if properly applied has given excellent
control of Pythium root rot of Chinese evergreen.
f. Terraclor (PCNB) has been effectively used for
the control of damping off caused by Rhizoctonia
in Pothos and Philodendron beds. Apply at the
rate of I to 1 1/2 lbs. per 1000 sq. ft.
g. Fungicidal drenches may be used as temporary
inhibitory soil treatment to prevent spread of a
disease. Ferbam, thiram, and captain are usually
applied at the rate of 1 tablespoon per gal. of
water per 10 sq. ft.
Treatment of containers
a. Best treated with soil.
b. Dipped in Formaldehyde (1 gal. Formalin in
18 gal. of water) stack wet and cover with
plastic film for 24 hours. Don't use for 4 5
c. Benches and flats can be treated with copper
naphthenate, 8 per cent concentrate diluted
( 1 gal. to 3 gal. of Stoddard solvent per
1000 sq. ft.). Dip or paint.
May injure seedling roots that come in
contact with treated wood.
Developing and maintaining healthy planting stock
For soil treatments to be effective it is necessary that disease-free
stock be planted in the treated soil.
Obtaining healthy stock
1. From a specialist propagator
2. From a few healthy plants available or taking tip
cuttings 12 inches are more above the soil.
3. By heat or chemical treatment of planting stock.
4. By repeated roguing of diseased plants, selection
isolation and strict sanitation.
Maintaining healthy stock
1. New stock brought in should be isolated.
2. Propagation operations should be isolated from all
possible sources of infection.
3. Basic stock should be maintained separately from the
1. Avoid handling treated soil unnecessarily.
2. Don't use untreated tools. Walk over or drag
hose across treated soil.
3. Avoid splashing infested soil into treated soil.
4. Never place plant parts, flats, or pots from treated
areas on the ground.
3/7/58 50 copies
Plant Pathology Department