Sorghum Diseases in India

Figure 10. Xanthomonas campestris pv
holcicola DNA EcoR1 restriction pattern. Pattern
which differentiates X. c. pv holcicola
from other pathovars is designated by arrows.
Lane 1, Xch isolate from Nebraska; 2, Kansas;
3-8, Lesotho; 9-10, Kansas; 11, Australia; 12,
Texas; 13,1 kb ladder size marker.
Identification of races of X. campestris
pv holcicola
Pathogenic specialization has been demonstrated
between other X. campestris pathovars
and their specific host cultivars. For example,
X. campestris pv oryzae exhibits race-specificity
on a differential set of rice cultivars containing
specific genes for resistance (Mew 1987). Other
examples include X. campestris pv malvacearum
on cotton and X. campestris pv vesicatoria on pepper
(Stall et al. 1986). To determine if X campestris
pv holcicola exhibited pathogenic specialization
on sorghum lines, 70 genotypes of sorghum
from Texas A&M University's All Disease
and Insect Nursery (ADIN) were screened for
resistance or susceptibility to six X. campestris
148
pv holcicola isolates [Xh 3 (Texas), Xh 112 (Australia),
Les 114, Les 143 (Lesotho), KS 93 (Kansas),
and Tx 4501 (Texas)].
Sorghum plants were grown in a greenhouse
(25-27°C day, 22-24 °C night) in square pots (4
inches on a side), one plant per pot, in bactopotting
soil. Plants were inoculated at 4 to 7 weeks
after sowing (approximately the four- to six-leaf
stage of growth). Bacteria were grown overnight
at 28 °C in nutrient broth shake cultures. Bacterial
cells were pelleted by centrifugation at
13 000 g for 10 min and then resuspended in
water to approximately 5 x 10 9 cfu mL -1 (50
Kletts). One leaf on each sorghum plant was infiltrated
with the bacterial suspension by a
Hagborg apparatus. The trial utilized a split-plot
design with one plant per replication and four
replications per treatment. Symptoms (watersoaking
vs no water-soaking) were determined
at 4 and 6 days postinfiltration.
Initially the 70 sorghum genotypes were
screened for their response to six X. campestris
pv holcicola isolates. Eight genotypes responded
differentially to one or more of the isolates; they
were screened again with a larger number of
X. campestris pv holcicola isolates. The pattern of
host responses is shown in Table 4. The recognition
of race specificity in X campestris holcicola is
important to sorghum breeders because it indicates
that more than one pathogen race is required
in screening for resistance to bacterial
streak.
Our data suggest that isolates of X. campestris
pv holcicola can be differentiated by their reaction
with different sorghum cultivars. Therefore,
it is likely that X. campestris pv holcicola exhibits
pathogenic specialization to sorghum. This is
significant in that (1) different sorghum genes
for resistance to X. campestris pv holcicola must
exist, (2) the potential for a large number of X.
campestris pv holcicola races exists, and (3) identification
of sources of resistance to X, campestris
pv holcicola will require screening with a mixture
of pathogen isolates.
Semiselective medium for X. campestris
pv holcicola
A semiselective medium has been adapted for
use in recovering X. campestris pv holcicola from
bacterial streak-infested plant tissue. The medium,
MXP (Claflin et al 1987) was initially de-