IMMUNITY TO PEA SEEDBORNE MOSAIC VIRUS: A REASSESSMENT

16 RESEARCH REPORTS

PNL Volume 13

1981

Hampton, R. 0.

and G. A. Marx

USDA, SEA-AR, Oregon State University, Corvallis, OR USA

NYS Agricultural Experiment Station, Geneva, NY USA

Immunity to pea seedborne mosaic virus (PSbMV) was reported to be con-

ferred by a single recessive gene, designated sbm (2). However, infected

susceptible plants show an array of symptom expression (4, 5), and plants

initially identified as PSbMV-immune have sometimes proved to be susceptible

just before plant maturity (Hampton, unpublished results). Some breeders

attempting to develop disease-resistant pea cultivars have therefore begun

to question whether PSbMV-immunity is indeed conferred by a single gene.

These circumstances prompted us to re-examine the genetic control of immunity,

Details of this study will be published separately, but here we wish to confirm

previous analyses (1, 2) and to discuss how the results might be applied in

breeding programs.

Four different USDA Plant Introduction (P.I.) accessions, three of which

were recently reported to be immune to PSbMV (3), were used as parents in

reciprocal crosses with a common susceptible line. The susceptible parent,

WL-1255A ^1/, was also homozygous recessive for wlo, the wax gene reported by

Gritton and Hagedorn (1) to be linked with sbm. Susceptible marker parent

WL-1183 reported by these workers, like WL-1255, also carried the edible-pod

gene, p_, which is linked with wlo on chromosome 6. One of our resistant

parents, P.I. 193586, had been included in the investigation by Hagedorn and

Gritton (2).

Our results clearly confirm those previously reported (1, 2), showing

that resistance is conferred by a single gene pair, linked with wlo on chromo-

some 6 (Table 1). Our segregation ratios were determined after the plants

were repeatedly inoculated with PSbMV and then rigorously examined for presence

of disease symptoms. Moreover, all progenies remaining symptomless for 60

to 90 days after PSbMV-inoculations were assayed repeatedly on Chenopodium

amaranticolor.

In view of the technical difficulties of screening for PSbMV-immunity

by conventional methods and of the demonstrated potential for mistaking tolerance

for immunity, it would seem desirable to consider the use of marker genes

closely linked to sbm as preliminary indicators of susceptible or immune

plants, in lieu of PSbMV-inoculation. Ideally, the marker gene used to identify

resistant plants would be visible in the seedling stage and be at least horti-

culturally neutral, if not beneficial. Although the waxless (wlo) plants can

be easily scored early in seedling development, they may be considered horti-

culturally undesirable because the waxless surfaces of the leaflets predispose

the plant to damage by post-emergence applications of herbicides now commonly

in use. Still, this problem ultimately may prove less serious than it pre-

sently seems. Newer herbicides such as trifluralin are applied as a pre-emergent

treatment and it is not yet known whether pea plants with wax genes are ad-

versely affected by herbicides applied in this manner. Nor is it known whether

wlo or other wax mutants are horticulturally undesirable in their own right,

i.e. in the absence of pesticides. Moreover, should the afila gene gain com-

mercial acceptance, cultivars with wlo may not be injured by herbicides since

wlo affects only the upper surface of the leaflets and since the leaflets in

af plants are converted to tendrils. Thus, it may be unwise to dismiss wlo

as deleterious without further evaluation.

1/ - Furnished by Dr. Blixt

PNL Volume 13

1981

RESEARCH REPORTS

Even if wlo should prove to be horticulturally undesirable, it still

might serve effectively, though indirectly, in developing a group of breeding

lines with resistance to PSbMV. In this approach a line carrying sbm and

wlo in the coupling phase would serve as a parent in crosses with a range

of susceptible breeding lines. Selection would then be practiced, as suggested

before, on the basis of the presence or absence of wax, i.e. discarding all

normal wax segregants and retaining waxless plants which also have horticul-

tural merit on other grounds. After further selfing and selection, a selected

number of wlo breeding lines would be tested by conventional means to confirm

the presence of sbm; crossover plants (Sbm-wlo) would be discarded. This

diverse group of breeding lines could then be used in a second round of crosses

with a line(s) carrying resistance in the repulsion phase, i.e. sbm Wlo.

Selections in this second round of crosses would again be practiced on the

basis of the presence or absence of wax as well as on horticultural merit,

but in this case the normal wax plants would be retained and the wlo segregants

would be discarded because all the progeny would be homozygous for sbm.

The second closely linked gene, p, (for edible poddedness), has the dis-

advantage of being an adult plant character. Gene p could nonetheless be used

in combination with sbm either alone or together with wlo, especially if the

objective were to develop edible-podded cultivars.

It must be emphasized again that breeders who take advantage of the

linkage wlo-p-sbm, either along the lines outlined above or in some other

way, should take the necessary precaution of testing their material for

immunity at crucial steps along the way.

1. Gritton, E. T. and D. J. Hagedorn. 1975. Crop Sci. 15:447-448.

2. Hagedorn, D. J. and E. T. Gritton. 1973. Phytopathology 63:1130-1133.

3. Hampton, R. 0. and S. W. Braverman. 1979. Plant Dis. Reptr. 63:95-99.

4. Hampton, R. o. 1980. PNL 12:27-28.

5. Hampton, R., G. Mink, L. Bos, T. Inouye, M. Musil and D. Hagedorn.

1981. Neth. J. PI. Path. 87 (in press).