PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
HEMI-
PARASITES
(contd)
Generally
not important
parasites in
Australia
WITCHWEEDS (Striga spp., Scrophulariaceae)
Have chlorophyll.
No true roots. Depends on their hosts for water, minerals and probably
some organic substances.
Stiff upright annual herbs up to 30 cm high (S. asiatica), flowers red,
yellowish or whitish, parasitic on the roots of many monocotyledons,
eg maize, sorghum, sugarcane, rice. Heavily infested hosts wilt.
Reproduces by seed. As many as 90,000 seed can be produced per plant.
Seeds need a resting period of 15-18 months before they germinate but can
remain viable for up to 14 years. Life cycle of 90-120 days.
Spread by contaminated crop (host) seed, forage, bags, containers, vehicles,
machinery.
The exotic witchweeds
are among the worst
weeds of the world,
causing up to 40 per
cent yield losses in
severely-affected
crops. Australia is the
only country in south
east Asia that is free
from the witchweeds
S. asiatica and
S.angustifolia.
Australia has native
witchweed species,
one of them has proved
a problem on Qld
sugarcane.
Mistletoes have a life
span of 20-30 years,
eucalypts may live
for >150 years
MISTLETOES (Loranthaceae, Viscaceae). There are about 70 species in
Australia but most do little harm. All species in Australia are native plants and
include include Amyema (commonly A. miguelii, A. pendula), Dendrophthoe spp. and
Notothixos spp.
Have chlorophyll. Mistletoes are more apparent on isolated trees or at the
edges of forests and in the higher branches of trees, and on stressed trees.
No roots. Depend on their hosts for water and all minerals.
Perennial shrubs, often pendulous, parasitic on upper stems of native
and exotic plants, eg eucalypts, conifers, wattles, birch.
Many species have attractive red flowers, leaves may mimic their hosts.
Spread by birds, eg mistletoe bird (Dicaeum hirundinaceum), and animals
which eat the seed and deposit them in their droppings. Seeds stick to the
host.
Damage. Can be serious pests of natural forests, plantations, orchards and
ornamental trees. A single mistletoe usually has little effect on a healthy
tree but if many mistletoes grow on one host, the tree may die as a result of
environmental stress and the mistletoe.
Mistletoe attachments on
a branch of silver birch.
PhotoCIT, Canberra (P.W.Unger).
380 Parasitic flowering plants