PESTS & DISEASES by
Shane Burns
Orchids
are subject to a number of pests & diseases - why?
-introduced with newly acquired or non-orchid companion
plant
-poor or erratic growing conditions within the
greenhouse
-weak & unhealthy plants as a result of poor
horticulture
pests
- an infestation i.e. the plant is attacked from
without
diseases
- an infection where the plant is attacked from
within
ailment
– a deficiency or excess of nutrients &
environmental factors
PESTS
generally
insects or mollusks pests
mites
spider mites, false spider mites
thrips
aphids
mealy bugs
scale insects
other insects - cockroaches, weevils, flies, millipedes
etc.
ants
slugs
snails
others
IDENTIFICATION OF PEST AND DAMAGE
Two spotted mite
- pale yellow/green, 2 dark spots on sides, 4
pairs legs, rounded
Damage -
feed on undersides of leaves, speckled appearance,
eventually turning black covered with silvery /bronze
webbing, curling under of leaves
False / red
spider mite
- white/yellow becoming red with age, 3 pairs of legs,
rounded
Damage -
feed on undersides of leaves, fine pitting turning to a
white/yellow flecking (dead cells) general discoloration
& leaf drop
Thrips
- yellow brown, elongate slender, moves rapidly
Damage -
deposit eggs in cells- watery bruise (magnified) of
stems & leaves, feed damage - glaucous or rusty spots on
leaves, turning black finely spotted transparent
or
blackish discoloration flowers, distorted buds
Aphids
- greenfly, green or black easily visible
Damage -
distorted buds, new growths, stems, spikes
Mealy bugs
- soft bodied covered in white powdery wax and
filaments
Damage -
hidden under dry bracts, sheaths & between roots, leaf &
stem axils. Suck plant sap, turns yellow
Soft or armoured scale
- flattened, rounded yellowish to black, only
nymphs mobile
Damage -
found on leaf surface, axils, hidden under bracts &
sheaths, on rhizomes. yellowing of tissue, leaf drop,
some blackening
Boisduval scale
- males in
cottony white masses, females whitish circular shields
Damage - in
leaf axils, hidden under sheaths and bracts. Yellowing
of tissue, some blackening, leaf drop
Cockroaches, crickets, millipedes, crustaceans
Damage -
eating root tips and flowers
Fungus gnats
- larvae of flies, adults minute black, larvae whitish
Damage -
seedlings, root tips, in decaying organic matter
Beetles, weevils, grasshoppers etc.
Damage -
eating of flowers & leaves or sucking/boring
Ants
- not
destructors but transporters of scale & mealy bug &
aphids . Feed on sugary exudations of insects. Secondary
infestation of black sooty mould
Slugs
- small charcoal, tigers, large yellows
Damage -
slime trails , nocturnal, moist, eating of roots,
flowers, new growths
Snails
- bush, conical, brown - charcoal, yellowish,
small shell for hibernation or prevent desiccation
Damage -
bush eat roots & stems, others mainly buds & flowers
CONTROL
Prevention
- cleanliness remove dead leaves & old
flowers,repot timeously, inspect regularly, act
immediately
Cure
-
insecticides, wettable powders, emulsions, liquid
concentrations
follow instructions implicitly
always wear safety clothing
spray in the late afternoon when cooler - dry by
nightfall
avoid watering for a day or two after spraying
repeat sprays at least twice, at 3-4 summer or 7-10 day
winter intervals to kill new born
full cover spray to point of drip
sticker or wetter ( sunlight liquid soap)
work backwards to exit
do not mix different chemicals
rotate chemicals to avoid resistance
Chemicals
Systemic -
taken up by plant thru leaves & roots
Contact -
taken up by insect thru spiracles or ingested through
eating
miticides
- mites
kelthane, temik, pentac, tedion,
emulsions
- waxy scales, thrips
oleum, sevin, malathion, rogor, metasystox
insecticides
– aphids: pirimor
- cockroaches, beetles etc baygon not on plants
- ants : diazinon, chlordane
molluscicides
- slugs & snails : metaldehyde, mesurol
Generally fungal, bacterial or viral
DISEASES
FUNGUS two
types cercospora sp and anthracnose
species
causes
spotting of the leaves and pseudobulbs, occasionally
flowers
rarely causes death, scars remain for life of leaf
more prevalent in cold, moist, & still conditions
especially active on devitalised and dead plants
Cercospora
symptoms
Chlorosis -
fine yellow spots starts on undersurface, first
enlarging irregularly-followed by necrosis, death and
blackening of tissue followed by irregular enlargement,
dead tissue falls out, leaves die
Anthracnose
symptoms
Brown
irregular discoloration usually sunken, sharply defined
between infected & healthy tissue, spore pustules -
little brown or black spheres develop (don’t confuse
with thrips droppings)
Botrytis
symptoms
flowers
only infected, small brown spots, enlarge very little,
pink margin
CONTROL:
Prevention
cleanliness and strict hygiene
adequate air movement
control of humidity levels and temperature
removal of leaves & especially flowers (botrytis is
saprophytic)
Cure
spray at regular intervals usually weekly
use a wetting agent
follow safety procedures as for pests
Chemicals
usually
wettable powders
benlate can cause viral like symptoms in excess
caftan, zineb, ferbam, daconjl, tersan, thiram, mancozeb
Fungal rots
can cause
death if not controlled
root & rhizome rots - rhizoctonia & fusarium
brown spot leaf rots - pseudomonas
black rots on pseudobulbs- phytophthora &
pythium
CAUSES &
SYMPTOMS
Black rots
- actually are fungi
prevalent in cool months & high humidity
leaf or new growth turn purplish brown surrounded by
yellow, quickly spreads to
rhizome and further growths, leaf may fall
at
slight touch
Root rots
affects
plants of any age
from broken down bark mediums
poor drainage
usually confined to roots except small
seedlings
wilting of the plants & shrivelling of
pseudoblubs
yellowing & twisting of leaves
Fusarium wilt
usually
confined to root & rhizomes
thru cut ends of divided plants
die within 3-9 weeks , may take a year
yellow thin shrivelled & twisted leaves
roots rotten
rhizome has purple ring & pink tissue in cross section
BACTERIAL ROTS
usually
cause death of plants
brown spots & rots pseudomonas & erwinia
Symptoms:
bacterial brown spot
- pseudomonas sp.
soft water
soaked lesion turning brown or black
advances rapidly
exudate on leaf containing bacterial
brown rot
- erwinia cypripedii - mostly in paphs
water
soaked spots near middle of leaf
spot darkens to chestnut brown
spots enlarging fairly rapidly
plants become shrivelled mass
soft rot
- erwinia carotovora - infrequent
outbreaks
enter
through wounds
wet rot with offensive odour
rapid spread in roots & leaves & new shoots
slower in pseudobulbs & rhizomes
AILMENTS
Due to
deficiencies or excesses of
nutrients
horticultural practices
environmental factors
WATER
Over
watering
- insufficient oxygen to the roots caused by too high a
water holding capacity of media
impervious
(to gaseous exchange) pots
over potting
drying prevented by insufficient air movement
Symptoms
- older leaves yellow & shrivel
- roots rot and die
Cure
- unpot , cleanup, high humidity, lower light
Under watering
insufficient moisture retention of media
rarely
inadequate frequency or amount
Symptoms
- shrivelling of plant
- brittle papery roots
Cure -
water lightly, second watering 1 hour later, may need a
third, soak individual plants up to 1hour
Water quality -
suitability of water
measured by electronical conductivity i.e. the soluble
salt content
HARD
WATER - high content of calcium & or magnesium salts
reduce misting or syringing - deposits on leaves
at
greater than 300 ppm, soap does not lather
Control
- acid media e.g. peat moss
-
fertilizers
with high residual acidity, leaching important
SOFT WATER
- high content of sodium salts
high ec ,
may be toxic to plants,
leaf & root tip burn
can use resinous demineralisers
LIGHT
measured in
foot candles, 1000 ft candles = 10% full sun
plants may tolerate higher light than that required
for
optimum growth
various species & hybrids have different requirements
&
tolerances
Excess
- tolerance dependant on facilities for dissipating heat
from leaves direct result from light
e.g. air
temp, humidity, air movement tolerance
sunburn due to perpendicular suns rays
pale green & dull leaves , pseudobulbs shrivel
Minimal
- due to excessive shading,
dark
green & weak spindly growth
blind growths reduced flowering
TEMPERATURE
Excess-
too high at night , poor flowering
growth poor as food used faster than made
leaves fall prematurely
Minimal
- water & nutrients absorbed slowly
yellow foliage & poor development control - can survive
short spells if assisted
reduce or increase temp differential, misting, shading,
humidity, air movement, heating
NUTRIENT DEFICIENCIES
ph
- the
acidity, i.e. low ph - below 7 or alkalinity , high ph –
i.e. above 7 affects the uptake of nutrients
- ideal usually between 5 & 6.5 - there are some
exceptions
- organic mediums generally self regulating so ph not so
important unless pebble culture, hydroculture used
- problems with nutrient uptake if water excessively
acid or alkaline
nitrogen (N)
- for leaf growth and shoot development
- deficient : stunted growth and mature too early old
leaves turn yellow and drop
phosphorus (P)
-as a catalyst for flower production and root
development
- deficient - stunted growth dark green leaves/ purple
tinge
potassium (K)
- for control of flower and fruit development
-
deficient - dwarfing & edges of leaves scorched & dead
calcium (Ca)
- for building cell walls & cell metabolism
- deficient - new growths stunted & distorted
magnesium (Mg)
- part of chlorophyll & food manufacture
- deficient - yellowing between veins, plants don’t
thrive
sulphur (S)
- an ingredient of proteins
- deficient - may stunt root growth
manganese (Mn)
- for cell activities
- deficient - poor growth ( trace element)
trace elements
-other chemicals Cu, Mb, B, Zn required only in minute
amounts- generally available in organic media & as
impurities in water & fertilizers
Excess - poisonous to plants, loss of new
growths,& chemical burn of mature parts
Deficiency - rare but generally poor growth
VIRUS
INFECTION
Types
- Odontoglossum ringspot / tobacco mosaic
- cymbidium
mosaic
Symptoms
- confusing
to professionals & amateurs alike
- appearance dependant on environmental
factors (light & temp) and genetic factors
(degrees of tolerance)
- appear as colour break streaking in flowers &
leaves and as necrotic lesions
Virus-like symptoms in leaves include chlorosis, spots,
dots, tip & marginal burns, scorching, water soaked
areas, green and yellow flecking - They may be due to
other causes
Transmission
- aphids, cutting tools, old re-used media,
unwashed pots,
Cure
-
apparently none, other than burning of infected plants.
With many thanks to my friend Shane Burns.
Uitkyk, Nelspruit, SOUTH AFRICA Tel: 013 747 2270
