ORCHID PESTS AND DISEASES

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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

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