Sorghum Diseases in India

Glume
Lemma-
Ovarian
locule
Nucellus
Lodicule
Figure 1. Diagram of sorghum spikelet at anthesis
showing the ovule, styles, lemma,
palea, lodicules, and glumes.
sues and the pericarp of the developing caryopses
during the next 4 weeks (Castor and
Frederiksen 1980). At physiological maturity,
colonies of fungi were observed in the starchy
mesocarp and the cross and tube cells of the
pericarp in all cultivars (Glueck and Rooney
1980).
Rain and warm temperatures following anthesis
increase fungal colonization of developing
caryopses (Castor and Frederiksen 1980;
Williams and Rao 1981). In mature grains, wet
weather and early fungal colonization increase
the rate and extent of deterioration (Glueck and
Rooney 1980; Williams and Rao 1981). One measure
of fungal metabolism is the presence of ergosterol
(Seitz et al. 1983). The ergosterol content
of grain and the rate of deterioration of mature
grain increased geometrically with degree of
rainy weather (Fig. 2). Hence, visual ratings may
not be a good predictor of fungal colonization of
the grain.
Inhibition by Phenolic Compounds
Resistant cultivars contain less fungi than susceptible
cultivars in the caryopses and glumes
(Castor and Frederiksen 1980; Forbes et al. 1989).
Resistant cultivars also respond more quickly to
fungal invasion via increased levels of phenolic
compounds and pigmentation of spikelet tissues
than do susceptible cultivars (Forbes et al. 1989).
266
Styles
Pedicel-
Glume
Sterile lemma
Palea
Ovule
Placenta
Vascular
bundles
400
300
200
100
0
1 2 3 4 5
Weathering scale
trend
Figure 2. The relationship of ergosterol content
in sorghum grain to a visual rating of
weathering on a 1 to 5 scale where 1 = less
mold and 5 = severe mold. Source: Data from
Seitz et al. (1983).
It seems that specific phenolic compounds in
spikelet tissues limit the growth of fungi.
Most cultivars with pigmented testa containing
polymers of flavan-3-ols (tannins) resist
weathering (Glueck and Rooney 1980; Hahn et
al. 1983; Bandyopadhyay et al. 1988). These cultivars
normally contain higher levels of phenolic
compounds, phenolic acids, and tannins than do
cultivars without pigmented testae (Hahn et al.
1984). Apparently, tannins or precursors of tannins
impart antifungal and antibacterial activity
in the developing and mature caryopses.
Cultivars with a red pericarp (with or without
a pigmented testa) containing significant
levels of flavan-4-ols exhibit resistance to weathering
[Jambunathan et al. 1986; Mukuru (this
publication)]. Cultivars with a white pericarp
without a pigmented testae do not contain significant
levels of flavan-4-ols; yet some of these
cultivars exhibit resistance to weathering. Apparently,
flavan-4-ols and related compounds
are involved in some way with resistance to
grain weathering.
Free phenolic compound and tannin contents
in caryopses increase significantly during development
(Doherty et al. 1987; Forbes 1986), reaching
maximum levels 7-18 days after anthesis
(Figs. 3, 4). At maximum, the levels of free phenolic
compounds and tannins were two to eight
times higher than those observed in the mature
grain. Phenolic compounds and tannins are apparently
being bound to cellular tissues, and
therefore are not extractable for analysis. The