PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
CONTROL
METHODS
Code for the
Control of
Branched
Broomrape
Some parasitic flowering plants can be difficult to control once established so early
detection should be followed by prescribed control measures. Some produce seeds
prolifically with a long dormancy period and viability, eg broomrapes. As crop
cultivation intensifies, parasitic plants are gaining significance as weeds. Also it may
be that only a certain species requires control.
For all these reasons, it is essential that parasitic plants be accurately identified
(pages 378, 412). For example, in states where branched broomrape (Orobanche
ramosa) is under eradication, suspect broomrape plants should be dug up with the host
still attached, and sent to the nearest office of Agriculture or one of the diagnostic
services set up to identify broom rape plants.
LEGISLATION
Native Vegetation Acts. Approval must be obtained before taking any activity
against native species, eg mistletoes (South New England Landcare, 2008).
Noxious Weed Acts (or their equivalent). Some parasitic plants are declared
noxious weeds, eg some dodders (Cuscuta spp.) in NSW, Vic., SA, WA and
Tasmania, witchweed (Striga sp.) in Qld, and branched broomrape (Orobanche
ramosa) in SA and are subject to obligatory control measures. Failure of
landowners to follow Codes and Protocols in some regions can lead to prosecution.
Quarantine Acts, eg all Orobanche spp. are prohibited imports.
Seeds Acts. Several States/Territories have regulations against the importation
and/or sale of crop seed infested with seeds of parasitic plants, eg dodder (page 386).
The Branched Broomrape Eradication Program is underway in some
states. Farmers want compensation for quarantine and a Quality Assurance scheme
to ensure the integrity of the eradication scheme.
CULTURAL METHODS.
Deep ploughing to bury seeds to a depth where they could no longer germinate
and infect their hosts and minimum tillage which exposes seeds to extremes of
temperature and moisture reducing crop infections, are not usually recommended
today. Dodder seeds can survive for 20 years.
Planting time. Higher density plantings can reduce witchweed plants perhaps due
to extra shading. Later plantings of some crops can reduce witchweeds and
broomrapes, due to lower soil temperatures but may also reduce potential crop yield.
Trap crops.
– Catch crops are susceptible plants grown on land kown to be
infested. They stimulate germination of witchweed seed and become infested
themselves. The crop must then be destroyed either by ploughing under or
applying herbicides before the parasite matures and sets seed.
– False hosts (decoy crops), eg non-host legumes, stimulate germination of
witchweed seeds which, however, cannot infect the false host and in the absence of a
true host starve to death. False hosts have been used in crop rotations to reduce
seed populations in soil but results have been disappointing.
– Variations. Flax can serve as a catch crop for broomrape. The flax root exudates
stimulate broomrape seed to germinate and these then infect the flax but broomrape
cannot flower on it.
– In the long term, the only option for severe infestations may be to switch to nonhost
plants.
Broomrape (Orobanche spp.)
– Heavy grazing by sheep if there is a history of infestation before planting crop.
– Cultivation of some, eg broomrape, can give some control if deeply buried.
– Flooding as for rice growing reduced infestation of broomrape (O. cernua) in
following tobacco crops. Seeds lose their viability after one month’s under water.
– Change of crop. In severe infestations of broomrape, the only options may be to
switch to non-host plants such as cereals, orchards or vines.
Dodder (Cuscuta spp.)
– Crop rotation. Use a non-susceptible rotational crop and control susceptible weeds.
When planting new areas, especially river flats, plant crops other than summer
growing legumes for 2-3 years before sowing lucerne to clean up possible dodder
and general weeds. Note that that it takes 10-20 years of fallow needed to deplete
dodder seed in soil. This is completely impractical.
Witchweeds (Striga spp.)
– Fertilizers. Witchweeds are frequently associated with infertile soils, especially those
deficient in nitrogen. Nitrogenous fertilizers are sometimes used to suppress
witchweeds but the precise process is not understood.
Allelopathy. Overseas Orobanche crenata causes huge damage to legume crops.
Field trials indicate that O. crenata infection of faba beans and peas is reduced
when these host crops are intercropped with oats. It has been suggested that this is
due to allelochemicals released by cereal oats inhibiting the germination of O. crenata
seeds, thus reducing infection of faba beans and oats.
Parasitic flowering plants 383