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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 molluscs
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
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
- 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
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
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)
-
flowers only infected, small brown spots, enlarge very little, pink margin
- cleanliness and strict hygiene
- adequate air movement
- control of humidity levels and temperature
- removal of leaves & especially flowers (botrytis is saprophytic)
- 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
- can cause death if not controlled
- root & rhizome rots - rhizoctonia & fusarium
- brown spot leaf rots - pseudomonas
- black rots on pseudobulbs- phytophthora & pythium
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
- 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 practises
- 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
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