Sorghum Diseases in India

Disease research conducted in Mexico during
the period 1980-87 is likewise presented.
Seedling Diseases
These types of diseases are caused mainly by
pathogens of the genera Fusarium spp, Pythium
spp, and Rhizoctonia spp. In the Bajio, most
farmers sow 22.5 kg hybrid seed ha- 1 . This
amount is above the average sown in other
areas, as a higher seedling rate is thought advisable
in view of losses to seedling decay, herbicide
residues (Dual), inefficient plowing, and
poor seed quality. Seedling diseases account for
20% of disease problems observed in some areas
of the Bajio, and farmers are advised to verify
the germination of the seed before sowing.
Downy Mildews
The most common mildews that occur in Mexico
are downy mildew (Peronosclerospora sorghi) and
crazy top (Sclerophthora macrospora). The latter
has no economic importance.
Downy mildew (P. sorghi) has spread to the
main grain sorghum production areas. In
Tamaulipas, it has been recorded up to 80% of
systemic infection on commercial hybrids
(Betancourt 1980). Conidial infection is very severe
at El Maluco, in the state of Michoacan,
with ratings of 2.5 on a 1 to 5 scale (Narro et al.
1983). It is important to point out that this species
attacks maize also, but in maize oospores
are not produced, as it is commonly observed in
sorghum. In commercial fields, effects on yields
of susceptible hybrids may be up to 10%. In experimental
fields, however, up to 50% yield loss
has been recorded when systemic infection
reaches 70% of incidence.
Pathotypes of downy mildew were identified
by using the Downy Mildew Virulence Nursery.
At Tamaulipas, pathotypes 1, 2, and 3 are present
(Aguirre 1984b). In Michoacan, Guanajuato,
and Jalisco only pathotype 1 has been found
(Narro et al.1982; Betancourt, in press). In Mexico,
pathotype 1 is the most common, averaging
40% of systemic infection in the hybrid ATx 399
x RT x 2536.
In relation to chemical control of downy mildew,
the fungicide metalaxyl (Ridomil ® ) (C.A.
48988) was evaluated in tests conducted at Mic-
78
hoacan in 1981. Results indicated that a dosage
of 1 g kg -1 of seed was very effective (Hernandez
et al. 1982). These treatments were also applied
to the hybrid Funk's G 766 W in commercial
fields at the Bajio with excellent control (N.
Avila, personal communication). For the most
part though, downy mildew control has been
through resistant hybrids. INIFAP released the
hybrids RB 3030 and RB 3006 for Tamaulipas
and Sinaloa (Williams 1980) and BJ 83 for the
Bajio (Narro 1984); each of these hybrids has
Tx 430 as common pollinator where a dominance
to partial dominance for resistance has
been found (Narro et al. 1982). So far, pathotype
3 is found only at Tamaulipas; no evidence of its
presence in other areas of Mexico is known. This
is fortunate, since RTx 430 shows susceptibility
to this pathotype. Nor have pathotypes 4 and 5,
identified at Brazil and the Honduras, been detected
in Mexico (R.A. Frederiksen, personal
communication).
The resistance observed in RTx 430 has been
very stable; in nurseries at Beeville, Texas, hybrids
with this pollinator show no more than 5%
systemic infection. Thus, RTx 430 may possess
several genes for disease resistance.
A program involving crosses of material developed
by INIFAP in Mexico and initiated in
1985 at Texas A&M University was selecting
germplasm adapted to temperate areas. Some F4
lines from this program show promise. Although
up to now we consider the sorghum
downy mildew (SDM) pathogen to be under
control, we need to develop new sources of resistance,
since the downy mildew pathogen has
the potential for change and can become very
destructive in a short time.
As mentioned before, crazy top is found in
very limited areas; in commercial fields it has no
economic importance.
Viruses
Of the 19 viruses registered worldwide as sorghum
pathogens (Toler 1980), only three are of
economic importance in Mexico: sugarcane mosaic
virus (SCMV), brome mosaic (BM), and
maize dwarf mosaic virus (MDMV).
The latter is the most important in the sorghum
production because it can infect up to
100% of susceptible material. Virus disease infections
of sorghum in Mexico were first re-