4. Physiological races of Puccinia sorghi in maize.

 

Some years ago, Vallega, (Anales del Instituto Fitotecnico de Santa Catalina 4:14‑16. 1942) made a preliminary report of the local Puccinia sorghi population, describing the presence of two physiologic races: A‑race which does not attack sugary corn line 13‑b coming from Urbana, Illinois, and B‑race which attacks that corn. Though Vallega's rust samples were not kept, the supposed A‑race was again easily isolated and is kept in our collection under number 1890. Later, we isolated three other physiologic races which have a differential behavior on the following maize lines:

 

13‑b sugary corn from Urbana, Ill., U.S.A.

14‑a  "

I.F. 3562 starchy corn from Cochabamba, Bolivia

I.F. 3861   "               La Paz, Bolivia.

 

We maintain Vallega's designation of B‑race, for that sample (Nr 1875) which attacks all differential varieties of corn. The remaining physiologic races behave as indicated in the following table:

 

 

 

Reaction of the different maize varieties

Rust sample No.

Rust race

14‑a

13‑b

I.F.3562

I.F.3861

 

1890

A

S

R

S

R

1875

B

S

S

S

S

1880

C

S

S

R

S

1884

D

S

SR

SR

S

 

S  = Susceptible (large pustules without a surrounding necrotic area)

SR = Semi-resistant (small pustules with necrotic area)

R  = Resistant (There is no pustule formation; only necrotic spots.)

 

The study of inheritance of resistance to A‑race (sample Nr 1890) in the cross of 13‑b with 14‑a maize has been started. The results of inoculations of parents, F1, F2 and corresponding backcrosses to each one of the parents are shown in the following table:

 

Generation

Reaction to rust A‑race

Total of inoculated seedlings

X2

P

Susceptible

Resistant

 

P... 13‑b

-

all

many

 

 

P... 14‑a

all

‑

"

 

 

F1

279

1

280

 

 

F2

306

105

411

0.06

0.8

(13‑b/14‑a)/14‑a

127

2

129

 

 

(13‑b/14‑a)/13‑b

63

68

131

0.37

0.7‑0.5

 

Evidently, resistance to A‑race in this cross depends on a single recessive gene, for which we propose symbol rp2. In the case of the original Rp gene located in chromosome 10 by V. Rhoades, resistance is dominant. Line 13‑b (su rp2) was crossed with a susceptible tester possessing g and Mr of chromosome 10. The F1 was completely susceptible and produced in F2 results indicated in the following table:

 

F1 genotype

=

+

 

g Mr

 

+

su

 

+ +

 

rp2

 

F2:

 

+

 

su

 

 

Mr

+

Mr

+

Total

+

g

+

g

+

g

 

g

 

+

rp2

+

rp2

+

rp2

+

rp2

+

rp2

+

rp2

+

rp2

+

rp2

 

198

51

85

33

110

26

3

1

71

24

26

6

23

10

0

0

667

 

Analysis of data of preceding table:

 

 

Segregation

X2

P

 

Rp2:rp2

1.98

0.20 ‑ 0.10

 

su : su

0.36

0.70 ‑ 0.50

 

Mr : mr

0.31

0.70 ‑ 0.50

 

G : g

1.30

0.30 ‑ 0.20

Linkage

Su:su and Rp2:rp2

0.66

0.50 ‑ 0.30

"

G : g and Rp2:rp2

1.26

0.30 ‑ 0.20

"

Mr:mr and Rp2:rp2

0.22

0.70 ‑ 0.50

 

There is no evidence of linkage either between rp2 and su of chromosome 4, or rp2 with g and Mr of chromosome 10.

 

R. H. Batallanez