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vitro to inhibit fungal growth. For example, 7-hydroxylated
simple coumarins may play a defensive role against parasit-
ism of Orobanche cernua , by preventing successful germina-
tion, penetration and connection to the host vascular system
(Serghini et al., 2001).
Weeds play one of the major roles in the loss of yield and
composition of medicinal and aromatic plants. The presence
of weeds during different periods of crop growth of cultivar
'Bourbon' resulted in decreases in linalool (9.6-11.4%), isom-
enthone (7.3-7.9%), citonellol (18.9-21.8%) and citronellyl
formate (5.5-6.6%), and increases in geraniol (23.5-26.5%)
and geranyl formate (2.2-3.0%). Rose oxides and 10-epi-
γ-eudesmol were not affected (Rao and Bhattacharya 1997).
Rao et  al. (2005) also reported the loss in yield and changes
in composition when the oil of three cultivars of rose-scented
geranium, namely, Algerian, Bourbon and Kelkar, was co-
distilled with companion weeds growing in the field. Water
requirement for the growth of weeds is primarily of interest
from the stand-point of competition with the crop plant for the
available moisture (Gibson, 2000). It has been reported that
wild mustard transpires about four times more water than a
crop plant (Thakur, 1984). Studies show that weed and canopy
architecture, especially plant height, location of branches and
height of maximum leaf area determine the impact of competi-
tion for light and, thus, have a major influence on crop yield
(Cudney et al., 1991). Members of the family Brassicaceae (such
as Coronopus didymus , a notorious weed in wheat crop) gener-
ally produce sulphur compounds such as glucosinolates. Allyl
glucosinolate is one of the predominant glucosinolates in many
brassicaceous species. In soil, this compound is hydrolysed into
allyl isothiocyanate, a volatile compound (Mayton et al., 1996),
which may be responsible for allelopathic interference.
Like other crops, medicinal and aromatic crops are sus-
ceptible to several pests and diseases which in turn drastically
reduce and deteriorate crop yield and composition. Rhizoctonia
solani has been reported to cause the leaf blight of C . forskohlii
(Shukla et al., 1993).
Fusarium solani, causing root rot of C. forskohlii, has also
been reported by Bhattacharya and Bhattacharya (2008). Leaf
rust, leaf spot, leaf blight and powdery mildew decreased
the concentrations of menthone (from 8.3% to 1.1-2.4%) and
isomenthone (from 4.2% to 2.0-3.4%), and increased the
content of menthol (from 84.1% to 87.0-90.8%), neomenthol
from (1.8% to 2.1-2.8%) and menthyl acetate (from 0.1 to 2.0-
4.3%) (Shukla et al., 2000). Wilt disease caused by Fusarium
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