RA 00015.pdf - OAR@ICRISAT

seeded pigeonpea, cv ICP-7332, also nodulates
well in agar deeps and we are now comparing
counts using Siratro and pigeonpea as the test
plant.
Using Siratro for counting, we found cowpeatype
rhizobia varying from less than 100 to more
than 10 5 /g soil within 0.1 ha during the dry
summer. Numbers also varied a hundredfold
within the top 30 cm of soil with little apparent
relation to soil moisture-one surface-soil
sample contained 10 5 /g but only 1.3 percent
moisture. Numbers increased tenfold after rain
in early May. The presence or absence of. a
pigeonpea crop in the previous season had little
effect on numbers. Such high populations will
make it difficult to introduce new strains by seed
inoculation.
Rhizobium Strains
From 160 strains of Rhizobium isolated from
soils in Andhra Pradesh and Maharashtra, 10
highly effective strains in nitrogen fixation were
selected for field-inoculation trials in Alfisols and
Vertisols. In pot trials with cv ICP-1, these
strains produced plants up to 120 percent heavier
at 6 weeks than the uninoculated control, which
received 240 ppm N in the nutrient solution. The
pattern of nodulation showed Rhizobium strain
effects as well as strain x host genotype interaction
effects. Some of the selected strains form
nodules containing a black pigment, and we hope
that this will be a useful marker in our inoculation
trials. At 60 days after planting only 16
percent of the nodules in Vertisol and 12 percent
in Alfisol contained this pigment in uninoculated
soils.
Nodulation and Nitrogen
Fixation
More than twice as many and much larger
nodules were formed during the rainy season in
Alfisol than in Vertisol, but after the rains nodule
formation in Vertisol was greater, so that by 120
days after planting there were 136 nodules per
plant in Alfisol and 200 in Vertisol. However, 96
percent of the nodules in Vertisol and 74 percent
of the nodules in Alfisol were hollow, apparently
the result of insect attack. The grub damage was
apparent from 30 days after planting, and was
more severe in the Vertisol plots. Such attack
could be a major factor limiting nitrogen fixation
by pigeonpea.
During the rainy season, seed inoculation with
Rhizobium in Vertisol significantly increased the
number and dry weight of nodules per plant and
plant weight at 45 days after planting, but no
effect was found at 80 days or on grain yield.
Where rice had been grown for several years,
the soil contained less than 10 3
rhizobia/g soil
and in this field pigeonpea responded to
inoculation.
Nitrogenase activity per plant increased from
16 µ moles C 2
H 4
/plant per hour at 30 days after
planting in Alfisol to 55 µ moles at 60 days and
decreased to 2 µ moles at 125 days. Nodules
recovered from Alfisol were more active in
nitrogen fixation than those from Vertisol, reflecting
differences in the amount of nodule tissue
per plant rather than differences in activity per g
nodule tissue.
Nodulation of Germplasm Lines
A preliminary survey showed a wide range in
nodulation of 731 pigeonpea germplasm lines
grown in the field. At 25 days after planting,
nodule number per plant varied among cultivars
from 5 to 48. By 55 days, the number varied from
10 to 64 per plant. The proportion of apparently
active pink nodules and senescent or damaged
nodules also varied a great deal and was not
apparently related to total number of nodules
per plant. For some lines, 40 percent of the
nodules were hollow at 55 days.
Screening for Disease and
Insect Reaction
Phytophthora Stem Blight
In the unusually wet August of 1976, a stem
disease developed which selectively killed cul-
81