PEA ENATION MOSAIC VIRUS: VARIATION IN RESISTANCE CONFERRED BY En

PNL Volume 15 1983 RESEARCH REPORTS 3

Baggett, J. R. and R. 0. Hampton Oregon State University

Corvallis, Oregon

Resistance to pea enation mosaic was reported by Schroeder and

Barton (2) to be conferred by a single dominant gene, En. They noted

that when resistant pea plants were inoculated with any of several pea

enation mosaic virus (PEMV) isolates, the plants usually became in-

fected, with only minor effects on growth. Hagedorn and Hampton (1)

demonstrated that commercial breeding lines putatively containing En

varied in degrees of PEMV-susceptibility, as indicated by symptom

severity. Even the most resistant lines, when inoculated in greenhouse

tests, either developed mild symptoms or, if affected by the virus more

severely, progressively recovered from symptoms. Follow-up field tests

confirmed that lines able to recover from PEM symptoms in the greenhouse

also manifested field resistance, corroborating the presence of the En

allele.

The data of Hagedorn and Hampton have been retabulated in descend-

ing order of susceptibility (disease indices), from greenhouse tests

(Table 1). To these data we have added a column to indicate presumptive

presence of the En allele, as deduced from greenhouse recovery data and

information provided by the breeder of the cultivar. The 45 entries in

Table 1 comprise a continuous range of PEMV responses including interm-

ediate types which, based on PFM- recovery, contain either En or en

alleles (see Line Nos. 15-25). These data suggsst that modifier genes

and perhaps cytoplasmic factors, working either independently of or

interactively with gene En, affect the type and severity of PEM

symptoms. Thus, among these intermediate lines, some with en showed a

lower disease index than some lines carrying En.

We sought to elucidate the nature of this modification by crossing

'Perfected Freezer 60' (En En) reciprocally with three susceptible

(en en) and three other resistant cultivars. F2 progenies from these

crosses were rub-inoculated in two successive "greenhouse tests, using

mechanically transmissible Wisconsin isolate (C3) of PEMV used by

Hagedorn and Hampton (1). These progenies were also field tested

Corvallis under a severe natural incidence of PEMV. Disease indices

were calculated from individual-plant symptom scores, where

visible symptoms, 1 = slight chlorotic flecking, 2 = moderate PEM

symptoms, 3 = severe PEM symptoms, and 4 = PEMV-induced plant necrosis.

To obtain a refined estimate of disease severity, we removed the p1ants

from containers at peak development of PEM symptoms and made side-by-

side visual comparison. This procedure, however, wa3 done at the

expense of greenhouse PEM-recovery data.

Symptom scores of greenhouse-grown progenies from PEMV-resistant x

susceptible parents were combined into resistant (scores 0 and 1) and

susceptible (scores 2-4) classes for computation of segregation ratios.

Most X values for 3:1 ratios were within a 0.05 level of probability,

clearly indicating conformity with and affirming single-dominant-gene

inheritance for PEMV resistance (Table 2). There were a few non-

conforming ratios. For example, control plants of 'Banff and

'Canner PL' tended to escape PEMV-infection during greenhouse inoculation,

particularly in Test #1, but were extremely susceptible in field tests

PNL Volume 15 1983 RESEARCH REPORTS

(Table 3). The cultivar 'Aurora' and the progeny of P.F.60 x Canner PL,

in Test #2, contained more than expected PEMV-susceptible plants.

Perhaps some of these plants were in the process of recovery when they

were sacrificed. Segregation ratios varied somewhat between progenies

of reciprocal crosses, with a suggestion of maternally mediated PEMV-

resistance by P.F. 60 in crosses with 'Melody' (en en), Aurora (En Ea),

and 'Tempter' (En En).

PEMV-resistance classes of naturally infected progenies (Table 3)

were derived by combining symptom scores 0-2 as resistant, and 3-4 as

susceptible. X² values for 3:1 ratios were within the 0.05 level of

probability for five of the six progenies from PEMV-resistant x suscep-

tible parents. Likewise, parental controls responded predictably to

natural PEMV infection. Maternal mediation of PEMV-resistance by

P.F. 60 was again suggested in progenies from crosses with Banff,

Aurora, and Tempter, particularly apparent in the distribution of plants

per symptom score.

Our results verify the work of Schroeder and Barton (2), but failed

to demonstrate germplasm-related modification of En. We expected

Aurora and Tempter to contribute germplasm that skewed progenies from

P.F. 60 toward greater PEMV susceptibility. Instead, these parents were

quite resistant to PEMV in our studies. Although it is tempting to at-

tribute PEMV-resistance enhancement to P.F. 60 cytoplasm, we recognize

that new approaches are likely to be required to further elucidate this

phenomenon.

1. Hagedorn, D. J. and R. 0. Hampton. 1975. Plant Dis. Reptr.

59:89 5-899.

2. Schroeder, W. T. and D. W. Barton. 1 958. Phytopathology

48:628-653.

lume 15

198'i

RESEARCH REPORTS

Table 1.

Ranking of pea lines and^cultivars by greenhouse indices for enation mosaic

symptom expression, 1973 .


No. Line	2 Source	Greenhouse index	Greenhouse recovery	Field index	En gene- present
1 72-1554	1	94	none	77	no*
2 Exp. 331-485	2	91	none	62	no
3 Exp. 326	2	86	none	80	no
4 9776	3	85	—		no#
5 Banff	4	85	—		no
6 72-626	1	85	f ev-none	63	no*
7 Melody	5	85	none	84	no*
8 72-665	1	82	none	46	no*
9 72-673	1	82	none		no*
10 Can. 49	4	80	none		no
11 Tonka	2	78	none	69	no
12 Dark Skin Perf.	-	76	none	83	no*
13 9778	3	74	—		no#
14 9868	3	74	, —		no#
15 Can. 69141	5	73	excellent		yes*
16 Anoka	2	71	none	37	no
17 Exp. 306	2	69	none		no
18 71-2687	6	61	excellent-fair		yes*
19 Aurora	5	61	good		yes*
20 9867	3	59	—		?#
21 9869	3	58	—		?#
22 Tempter	5	53	good		yes*
23 72-3711	6	47	good, fair		yes*
24 Puget	8	47	none, fair		no*
25 72-689	1	46
26 70C-56	6	39	good	11	ves*
27 Perf. Fr. 60	-	38	excellent	6	yes
28 71-GP106	6	36	good		yes*
29 Freezer 52	4	35	good, excellent		ves
30 226 C	7	35	good		yes
31 328 F	7	34	good		yes
32 70C-36	6	31	good	6	yes*
3"J Trident	8	31	excellent, fair		yes*
~t4 Fr. 6650	5	31	excellent	5	yes
35 310 F	7	30	good	2	yes
36 Can. 695	5	29	excellent	3	yes
37 H286-1-1	8	28	excellent	1	yes*
38 329	7	27	excellent		ves
39 H294-5-1-1.	8	24	excellent	0	ves*
40 H302-2	8	24	excel lent		yes*
41 203 C	7	24	good	7	yes
4 2 Fr. 50	4	24	excellent	5	yes
43 Can. 50	4	22	excellent		yes
44 H312-2-3	8	21	excel lent	2	yes*
45 305 C	7	21	good	10	yes

1- From ihe data of D. J. Hagedorn and R. 0. Hampton (Plant Disease Reporter 59,

No. 11, 8M5-399. 1975.)

2- Sources: (1) Anonymous, (2) Northrup King, (3) Canner Seed, (4) Pureline,

(5) Rogers, (6) Crites-Moscow, (7) Western Valley, (8) Gallatin Valley

3- Concluded frorr. test results, with emphasis on greenhouse recovery tendency.

* indicates the breeder confirms conclusion. # indicates that one parent was

enation mosaic resistant but progenies were not selected for resistance.



	6 PNL Volume 15 1983 RESEARCH REPORTS

	Table ft Ratios of resistant:susceptible plants and disease indices, sreenhouse tests, 1978.



	•Significant at 5% level. ,Ratios of R:S for reciprocals compared by contingency table test. 'Complete distribution of reciprocals in disease classes 0-4 compared by contingency table test.

	Table 3. Ratios of resistant:susceptible plants, and disease indices, field test, 1978.



	"Significant at the 5% level. ,Ratios of R:S for reciprocals compared by contingency table test. "Score distribution for reciprocals compared by contingency table test.