Sorghum Diseases in India

greenhouse tests. Furthermore, he observed a
disease complex between Macrophomina phaseo-
Una and P. hexincisus; the effects of the two
pathogens on sorghum were additive. Both Cuarezam-Teran
and Trevathan (1958) and Motalaote
et al. (1987) reported that at initial
populations of ca. 500 nematodes L -1 of soil,
P. zeae would suppress growth of sorghum in
greenhouse tests. Motalaote et al. (1987) also
showed that the relationship between initial
nematode populations and sorghum growth response
fits a quadratic model (Fig. 1).
6
5
4
3
2
1
y = 4.07 +0.53 x-0.11x 2
(P = 0.001, r 2 = 0.67)
2 4 6 8 10
LN (PI+1)
Figure 1. Growth response of sorghum cv Pioneer
8222 to infection by Pratylenchus zeae
in greenhouse tests (reprinted with permission
from Matalote et al. 1987).
in a comparison of sorghum response to infection
by P. brachyurus and P. zeae, these
workers noted that sorghum growth was unaffected
by initial populations of P. brachyurus as
high as 8000 individuals L -1 of soil in greenhouse
tests. They also noted that sorghum was a
relatively poor host for P. brachyurus and a relatively
good host for P. zeae. They suggested that
the rapid and extensive necrosis of sorghum
seedling roots was a hypersensitive-type reaction
which limited reproduction by P. bra-
chyurus. Pratylenchus zeae, in contrast, induced
little necrosis of sorghum roots. In a separate
study with different sorghum cultivars, Starr et
al. (1984) reported that P. zeae did cause significant
necrosis of sorghum roots, and that the hydrogen
cyanide potential of infected tissues was
reduced. Motalaote et al. (1987) also reported evidence
for variation among sorghum lines in
ability to support reproduction of P. zeae; even
though all sorghum lines tested were hosts,
there were significant differences in P. zeae reproduction
among the different lines.
Johnson and Burton (1973, 1977) observed
highly significant forage yield increases of pearl
millet and sorghum/sudangrass hybrids in response
to nematicide treatments. These tests
were conducted in fields infested with a polyspecific
nematode community, so it was not possible
to identify which of the nematode species
were actually responsible for the damage to the
crops. However, the authors did suggest, based
on observed nematode population dynamics,
that P. brachyurus and P. zeae were causally involved
in the suppression of crop growth.
Ectoparasites
As with the other nematode groups, several species
of nematodes with ectoparasitic feeding
habits have been associated with sorghum. In
some cases it is difficult to assess the role of the
ectoparasites because the data are from field
tests involving polyspecific nematode communities,
as was the case with the work of Johnson
and Burton (1973,1977). Trichordus christiei, Criconemoides
ornatus, Belonolaimus longicaudatus
and Xiphinema americanum, in addition to endoparasitic
Pratylenchus species, infested the weedfree
plots in these tests. All nematode species
exhibited population increases, showing sorghum
to be a host for these species. However,
based on the relationships between nematicide
treatments, yield responses, and nematode populations,
Johnson and Burton (1973) concluded
that B. longicaudatus was the only ectoparasite
present that was pathogenic on the sorghum.
The Pratylenchus species present were also
pathogenic.
There is more concrete evidence of pathogenicity
for other ectoparasitic species. Three
species of stunt nematodes have been reported
to be pathogens of sorghum. In 1977, Smolik re­
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