PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
BIOLOGY & IDENTIFICATION
Parasitic flowering plants
NO. SPECIES
IN AUSTRALIA
SOME
DISTINCTIVE
FEATURES
WEED STATUS
OF PARASITIC
PLANTS
BENEFICIAL
VALUES
IDENTIFICATION
There are more than 100 species in Australia, but only a few are important weeds.
PARASITIC FLOWERING PLANTS PRODUCE FLOWERS AND SEEDS
similar to those produced by the plants they parasitize.
They belong to several widely separated botanical families.
Their parasitism is generally regarded as a degenerative process whereby plant
species became dependent for their existence on the host plant.
They have developed specialized organs which penetrate the vascular tissue of the
host plant, and absorb nutrients and/or moisture from it.
PARASITIC PLANTS VARY IN DEPENDENCE ON THEIR HOST PLANTS.
However, there are, in general, 2 groups:
Hemi-parasites possess chlorophyll and can carry out photosynthesis, some
possess roots while others do not, eg
– Native cherry (Exocarpos cupressiformis) - root parasite
– Western Australia Christmas tree (Nuytsia floribunda) - root parasite
– Witchweeds (Striga spp.) - root parasites
– Mistletoes (Amyema spp., Dendrophthoe spp., Notothixos spp.) - stem parasites
– Quandong, sandalwood (Santalum spp.) - root parasites
– Devil’s twine, dodder laurel (Cassytha spp.) - stem parasites
True parasites lack chlorophyll and have no true roots, eg
– Dodder (Cuscuta spp.)
– Broomrape (Orobanche spp.).
THE EFFECT OF A PARASITIC PLANT ON ITS HOST IS VARIABLE, eg
Relatively few of the known parasitic higher plants cause important diseases of
agricultural crops or forest trees.
Some, eg dodders, branched broomrape, witchweed, are declared noxious weeds in
some areas of Australia (pages 412, 417).
Large infestations of mistletoe can kill trees.
Broomrape and dodder infestations can dramatically reduce crop yields.
FOOD POTENTIAL, OIL, CEREMONIAL, BIO-CONTROL AGENTS, eg
In Australia some parasitic plants produce edible fruits, eg mistletoes, quandong
(Santalum acuminatum) and yellow plum (Opilia amentacea).
Sandalwood oil from Santalum spp. (root parasite of trees).
Some have ceremonial use, eg mistletoe and love.
Devil’s twine (Cassytha pubescens) has potential to reduce gorse infestations.
Some, eg mistletoes, provide habitat and food for many birds and mammals.
Honeyeaters feed on nectar in mistletoe flowers.
IDENTIFYING THE PARASITIC PLANT
This is an essential 1 st step in understanding its biology, impact and control.
To the average gardener and grower some parasitic plants, eg dodder and devils
twine, can look alike, and the actual species of a parasitic plant can be even more
difficult to identify (weed identification, page 412).
HELP WITH IDENTIFICATION
Most botanic gardens and State diagnostic services can assist (page xiv).
There are free specialist diagnostic services in some areas for some species, eg
broomrape in WA (Grain Guard or AGWEST Plant Laboratories).
Check with the diagnostic service on how to submit the specimen, eg leave
broomrape attached to the host if possible, to aid identification.
CONFUSION
Do not confuse stem parasites with non-parasitic plants which may:
Just twist their way around plants, strangler-type plants, eg jasmine.
Produce suckers on their stems and attach themselves to fences, buildings, other
plants, eg some ivies.
Produce tendrils which twine around other plants, fences etc, eg Sollya.
Be epiphytes, which are plants which grow on other plants and use them mostly
for physical support and protection. They are not parasites and cause no harm
to the plants on which they grow, eg most orchids in tropical areas which have both
chlorophyll and aerial roots.
378 Parasitic flowering plants