PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
NON-SELECTIVE & SELECTIVE HERBICIDES (contd)
Broad & narrow spectrum herbicides
WHY ARE SOME
HERBICIDES
SELECTIVE?
Selectivity may be achieved in many different ways, eg
Physiological differences between weeds and desired plants., eg
– Rate of herbicide uptake by roots.
– Rate of herbicide movement in the phloem (food conducting) or xylem (water
conducting).
– Different rate of herbicide breakdown in the plant.
– Different degree of herbicide tolerance, eg this may be due to the physical
characteristics of the leaves such as cuticle penetration. Other herbicides may
interfere with photosynthesis or respiration in certain plants.
– Some herbicides affect chemical processes within the weed but not the crop.
– Breakdown of herbicide by some plants and not others, eg maize, but not some
weeds, can metabolize atrazine to a non-toxic compound so that maize is not
killed but the weeds are.
– Much of their selectivity depends on their rate of absorption through the leaf
surface. They generally are more likely to injure plants if they are absorbed
rapidly. Selectivity is determined by the ability of the plant to either tolerate
the herbicide or break it down to harmless substances as it is absorbed.
Leaf structure., eg
– Leaf area (narrow or broad leaf), eg rosette broad leafed weeds catch and hold
more spray than grasses.
– Leaf arrangement (open or closed).
– Nature of leaf structure (hairy, waxy, etc).
– Location of growing point (exposed, protected).
Large leaves catch
hold more chemical
Small leaves catch
and hold less chemical
Open arrangement
of leaves, good
leaf contact
Restricted
arrangement,
of leaves, poor
leaf contact
Smooth leaf,
herbicide
runs off
Rough leaf,
herbicide
retained
Multi-channel leaf,
herbicide caught
in hollows
Growing point
protected
Growing point
exposed
444 Weeds - Integrated Weed Management