Common root rot of pea caused by the soil-borne fungus Aphanomyces euteiches is a serious problem in certain temperate regions where peas (Pisum sativum) are intensively cultivated. The characteristic symptom of the disease is a watery rotting of the root system, together with the presence of oospores of the fungus in the root bark [8, 12]. This disease occurs in North America, Europe, Japan, Australia and New Zealand [2]. Investigations in Sweden have registered the widespread occurrence of the fungus in soils from pea growing areas [1, 3-6, 10]. Furthermore, the investigations by Engqvist [5] and Engqvist and Ahvenniemi [7] revealed interesting interactions between pea genotypes and environments with different degrees of infection by A. euteiches. The aim of this study was to determine whether the fungus was present in the official pea cultivar trials in northern Sweden and, if so, to study the effect of the presence of the fungus on crop yield.

Material and Methods

Soil samples were collected from randomly selected locations of official pea cultivar trials in Sweden. Other soil samples, mainly from pea growers who wanted to test their fields prior to sowing, were also tested. All soil indices reported in this investigation are greenhouse indices determined according to Sherwood and Hagedorn [13], as modified by Engqvist [5]. The soil indexing method for determination of the degree of infection is based on expression of symptoms caused by A. euteiches on pea plants grown in representative soil samples from pea fields. The index is presented on a scale of 0 to 100, where 0 = no detectable presence of the fungus (i.e. no disease), 100 = very severe disease, and intermediate values indicate intermediate infection. The index is designated A.e.-index in the tables. The soil indices were determined on 4-liter soil samples using cultivar Bodil as the baiting plant. Six cultivars and breeding lines of dry field peas were included in the interaction studies. Protein content was calculated as 6.25 ? N-content. N-content was determined by the Kjeldahl method. Cultivars Timo, Bodil, Rigel and Capella and lines Hja 51850 and Hja 52259 from the official pea cultivar trials in 1988 and 1989 were tested in northern Sweden for seed yield. The trials were grouped according to the severity of Aphanomyces infection. The tolerance of a pea cultivar to root rot can be calculated as the relation of the yield in root-rot-infected trials divided by the yield in trials without root-rot infection (last column of Table 2). Statistical analysis was conducted using Student?s t-test. Differences with P values <0.05 were considered significant.

Results

Occurrence of the fungus

The 1984-1998 results of soil tests by Svalof Weibull AB indicate the occurrence of common root rot of peas in Sweden. Samples were taken from fields which were intended to be sown with peas. Common root rot was found in 34% of the soil samples originating from the official pea trial locations. The fungus was found in 18 of 22 counties where samples were taken. The degree of infection ranged from 0 to 100%. The 328 soil samples from the official trial locations had an average greenhouse index of 11, while 9% of the soil samples from the official trials had a greenhouse index which exceeded 50.

Influence of A. euteiches infection on yield

Seed yield was significantly reduced when peas were grown on root-rot-infected fields. The mean reduction over two years for the cultivars and lines was 708 kg/ha or 21% (P < 0.001). When peas grown on healthy and infected ground were compared on a year by year basis, the significance of the reduction was verified in one of the years when the A.e.-index was higher, whereas another year with a low A.e.-index the reduction was not significant (Table 1). The reactions of individual cultivars are presented in Table 2. Best over all performance was shown by Capella with an average seed yield of 3468 kg/ha. Timo showed the best resistance to A. euteiches in this investigation by only reducing its yield by 5% in trials with infection compared with trials without infection. Bodil showed high sensitivity for A. euteiches by reducing its yield in trials with infection by 39% (1401 kg/ha) compared with trials without infection.

***P < 0.001

Influence of A. euteiches on protein content

The protein content of peas from fields infected with the fungus A. euteiches tended to decrease, when compared with the protein content of peas from fields without infection. In the year 1988, when the average root rot potential was higher, there was a higher tendency for reduction of the protein content of the peas than in 1989 with a lower root rot potential (Table 3). It should be emphasized that these differences were not significant when analyzed with Student’s t-test.

Influence of A. euteiches on thousand kernel weight

The thousand kernel weight (TKW) of peas from fields infected with A. euteiches showed a significant increase, when compared with the TKW of peas from fields without infection. The average increase in TKW for the years 1988 and 1989 was 25 g (P < 0.05). In the year 1988, when there was a higher root rot potential, there was a higher increase in TKW than in 1989 with a lower root rot potential. All cultivars tended to increase their TKW in infected fields. The range of increase was 3 to 41 g. Of the cultivars studied, Capella was the least sensitive in this character (Table 4).

Influence of A. euteiches on hectoliter weight

The hectoliter weight of peas from fields infected with A. euteiches showed a significant increase, when compared with peas from fields without infection. The average increase for the two years was 4.5 kg (P < 0.001). The significance of the increase was also verified when peas grown on healthy and infected ground were compared on a year by year basis. There was a higher increase in 1988 when the root rot potential was higher than in 1989. Cultivar Capella had the highest increase of the hectoliter weight on infected land of the cultivars tested.

Discussion

A comparison between the interactions studied at sites further south in Sweden [5], in Finland [7], and those in the present investigation at sites in northern Sweden showed that the influence of common root rot of pea on yield and protein content was very similar. The results indicate the widespread incidence of A. euteiches in field soils in the important pea-growing areas of Sweden. The average degree of infection observed in this investigation (A.e.-index 22 based on 2852 soil samples) has, however, not changed compared with an investigation studying 849 soil samples during 1984-1990 [5].

Marx et al. [9] studied the genetics of tolerance in pea to common root rot. The genetic basis for tolerance to common root rot found in a selection from P.I. 175227 was studied in a greenhouse tank test. Parental, F1, BC, F2, and F3 populations were screened by subjecting the seedlings to a pure culture of the pathogen. The results showed that tolerance was associated with the dominant, "wildtype", alleles at three unlinked marker loci : Le (tall plant), A (colored flower) and Pl (black hilum) and that substitution of the recessive, horticulturally desirable, allele at each of these loci led to a decrease in tolerance. Shehata et al. [11] reported a complex pattern of inheritance with no evidence of dominance.

The cultivar Timo used in this present investigation is Le (tall, about 100 cm) and A (colored flower), and the better tolerance of Timo over the other cultivars studied, which were all le and a, is consistent with the finding of Marx et al. [9]. However, in a more comprehensive investigation at sites in south and central Sweden [5], the white colored pea cultivar Capella showed 10% better tolerance to common root rot than the colored flowered (A) pea cultivar Timo.

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