Ecology and management of an exotic weed Phalaris minor - Invasive Plants: Ecological and Agricultural Aspects - page 163

minor in wheat for the last 30 years [30]. Continuous and excessive use of iso-
proturon has resulted in selection for resistant biotypes of P. minor [1]. Sharma
et al. [31] found that isoproturon at 0.5-1.5 kg ha -1 applied pre-emergence sig-
nificantly inhibited seedling emergence of a Delhi biotype but not a Haryana
biotype of P. minor . Delhi biotypes were completely killed by isoproturon at
1.5 kg ha -1 applied pre-emergence and at all post-emergence doses. However,
the Haryana biotype was only killed at the highest post-emergence dose of
1.5 kg ha -1 . Resistant P. minor biotypes in Punjab were not controlled by dou-
bling the recommended dose of isoproturon to 1.9 kg ha -1 , which was phyto-
toxic to wheat [32].
P. minor biotypes in the Middle East developed resistance to fenoxaprop-P
[33]. Resistance of P. minor to isoproturon in India was reported in 1990. Since
then, several alternative herbicides are recommended for P. minor control,
such as clodinafop, fenoxaprop-P, sulfosulfuron, trifluralin (2,6-dinitro- N,N -
dipropyl-4-(trifluoromethyl)benzenamine), diclofop (2-[4-(2,4-dichlorophe-
noxy)phenoxy]propanoic acid), 2,4-D ((2,4-dichlorophenoxy)acetic acid),
metribuzin (4-amino-6-(1,1-diethylethyl)-3-(methylthio)-1,2,4-triazin-5(4 H )-
one), and tralkoxydim (Tab. 2).
Following the reports of isoproturon-resistant biotypes of P. minor in 1990s,
the list of alternate herbicides recommended for its control included
fenoxaprop-P. Later, Heap [33] reported detection of fenoxaprop-P resistance
in P. minor . Thill and Lemerle [34] discussed world wheat and herbicide
resistance development. They state that while trifluralin may injure wheat,
chlorotoluron ( N '-(3-chloro-4-methylphenyl)- N,N -dimethylurea) and tralkoxy-
dim may be useful for P. minor management in wheat.
Kaur et al. [35] conducted glasshouse experiments to explore the potential
of isoxaflutole (5-cyclopropyl-4-isoxazolyl)[2-(methylsulfonyl)-4-(tri-
fluromethyl)phenyl]methanone) to control P. minor and reported that a con-
centration of 0.5 mg L -1 reduced P. minor shoot height without affecting
wheat. Also, isoxaflutole did not alter pH, organic matter, and macro or micro
inorganic ions. However, this response may be restricted to soil type and dose
of herbicide used in the study. Isoxaflutole and metsulfuron (2-[[[[(4-methoxy-
6-methyl-1,3,5,triazin-2-yl)amino]carbonyl]amino]sulfonyl]benzoic acid)
may be better herbicides than atrazine (6-chloro- N -ethyl- N '-(1-methylethyl)-
1,3,5-triazine-2,4-diamine) for P. minor control (Fig. 3), field trials are needed
to substantiate the glasshouse results.
Resistance of P. minor to isoproturon is governed by an increased activity
of cytochrome (Cyt) P450 monooxygenase enzyme. Singh et al. [36] studied
the effect of the monooxygenase enzyme inhibitor, ABT (1,aminobenzotria-
zole), on the activity and rate of degradation of isoproturon in resistant and
susceptible biotypes of P. minor . The resistant biotypes had higher activity of
enzymes responsible for isoproturon degradation.
Questions that need to be addressed are: i) why does continuous herbicide
use, e.g., isoproturon, lead to high activity of Cyt P450 monooxygenase, and
ii) how many monooxygenase enzymes are involved in isoproturon degrada-