The response to crop health and productivity of field pea (Pisum sativum L.) at different growing conditions

ABSTRACT With increasing the area of legumes one of the most possible risks for productivity limitation is diseases. The research on the response to diseases and productivity of field peas was carried out during 2018–2020. The six cultivars and breeding lines were grown at two different infection levels: at natural field infection and under disease control using seed treatment and foliar fungicides. The main disease in field peas during the research year was Ascochyta blight at different intensites depending on year, cultivar/breeding line and disease control efficacy. Dominating pathogenic fungus D. pisi on harvested grains prevailed. Pea grain yield was significantly affected by cultivar/breeding line, experiments year and growing conditions. The highest yield difference between growing conditions (natural field infection and under disease control) was recorded in 2020 when Ascochyta blight and Grey mould gave the most severe attack. This finding illustrates the importance to eliminate one of the most important limiting factors for productivity – severe diseases. Future research on the forecast system of Ascochyta blight and other field pea diseases infection risk is needed. The response of cultivar/breeding line to weather conditions was established in this research as well. Tested breeding lines showed higher drought stress tolerance compared with commercial cultivars. More focus on environment stress-resistant cultivars is needed.


Introduction
Field pea (Pisum sativum L.) is an important leguminous plant in the structure of crops (Sainju et al. 2019). The increasing need for high-quality protein and the important role of this crop in rotations is driving interest in this crop. Also increasing interest in field peas as a component of intercrops (Lake et al. 2021). Pea is a perfect choice when it comes to the Farm to Fork Strategy because locks nitrogen in the soil and doesn't need chemical fertilisers, contributing to greenhouse gas emission targets. With Greening requirements in 2015 area of peas in Lithuania has grown exceptionally from 24 thousand ha in 2012 to 148.7 thousand ha in 2016. With such a significant increase in area, the need for locally adapted cultivars, the new knowledge about cultivation technologies, risk of diseases and pest had increased. Among the diseases, root rot diseases caused by Pythium ultimum, Rhizoctonia solani, Fusarium oxysporum f. sp. pisi Fusarium solani and Aphanomyces euteiches are reported as important peas pathogens from all commercial pea growing areas of the world (Khan et al. 2016). Leaf and stem diseases of peas caused by the Ascochyta complex, Peronospora viciae and Erysiphe pisi are frequent in most pea-growing regions (Bretag et al. 2006;Lake et al. 2021). Ascochyta blight is caused by a complex of fungal pathogens. The most devasting host plant is Ascochyta pisi Lib. (teleomorph: Didymella pisi sp. nov.). This pathogen causes leaf, stem and pod spot. The second very important Ascochyta complex pathogen is Ascochyta pinodes L.K. Jones (teleomorph: Mycosphaerella pinodes (Berk. &Blox.) Vestergr.). This pathogen causes foot rot and leaf, stem and pod spot also. Phoma pinodella (L.K. Jones) Morgan-Jones & K.B. Burch symptoms can be identified on leaf, stem and foot rot (Davidson et al. 2009;Liu et al. 2013). The majority of mentioned diseases are highly significant in our region as well (Gaurilčikienė et al. 2008;Marcinkowska et al. 2009;Česnulevičienė et al. 2014;Brauna-Morževska et al. 2019). Breeding on yield limitations factors such as diseases and abiotic factors play a significant role in improving field pea yield. The impact of these factors varies with agroecology and crop management practices and disease control as well.
The increasing requirements for the reduction of plant protection products and improvement of crop rotation structure with sustainable plants determine the increase in the area of leguminous plants, but obligates to manage effectively the mentioned risk factors limiting productivity.
The aim of this study was to evaluate the influence of peas cultivars and chemical control on pea productivity in managing diseases from early stages to harvest.

Materials and methods
The research was carried out at the Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry, Akademija, Kėdainiai District in Lithuania during 2018-2020.
The field experiments were established during three seasons. Three commercial cultivars (Astronaute, Jūra DS and Ingrid) and three breeding lines (3637-2 (Lina DS), 3784-1 and 3795-3 (Egle DS)) of field pea were grown. The field experiments were laid out at two different infection levels: natural field infection and under disease control using seed treatment and foliar fungicide.
For seed dressing commercial seed treatment fungicide, Maxim 025 FS (fludioxonil 25 g l −1 ) at a rate of 2.0 l t −1 was applied. For foliar application fungicide, Propulse (125 g l −1 fluopyram and 125 g l −1 prothioconazole) was used at a rate of 0.8 l ha −1 .
The field experiments were established in a randomised complete block design with four replications. Each treatment was allocated randomly within the block and replicated four times.
The crop was grown according to the local agronomic practice. For soil tillage, ploughing has been used in autumn. Base fertilisation of nitrogen at dose N30 was done in spring before drilling each research season. Phosphorus and potassium doses were calculated every year according to soil analysis data and needs for grain yield. 1.0 million seeds ha −1 were sown. A pre-emergence herbicide Fenix® (SC) with the active ingredient aclonifen (600 g l −1 ) for the control of annual weeds was used during all three seasons. Insecticides against insect pests were applied according to local thresholds.
The field experiments were established in a randomised complete block design with four replications. Each treatment was allocated randomly within the block and replicated four times.

Assessments in the field experiments
Assessment descriptions from EPPO standards 'Leaf and pod spots of pea' PP 1/172(2) and 'Seed treatments against seedling diseases (trials under controlled conditions)' PP 1/125(4) were used.
For Ascochyta leaf assessment: on 25 stems, randomly distributed throughout the plot, the percentage of leaf area infected was estimated.
For Ascochyta and Botrytis pod assessment: 100 pods randomly distributed throughout the plot, estimated the percentage of surface infection on each pod.
For seedling blight and root rot infection: Twenty-five plants were randomly sampled from each plot at the early growing stage (BBCH 15) and at beginning maturity (BBCH 79-81) to set up a steam base infection and determine the incidence and severity.
Scale 1-5 scoring assessment was used. The scale was converted to disease index using the following formula: , c, d, e = number of plants in groups 1-5 as indicated from the assessment.
The index values (x) can vary from 1 to 100. At full maturity (BBCH 89), plots were harvested with a plot combined Wintersteiger Delta. After harvest 1.0 kg sample from each plot was taken for grain analysis. The infection level of harvested grains was established.

Laboratory analyses
The agar plate method (Mathur and Kongsdal 2003) was used for the determination of infection levels with pathogenic fungi of harvested pea grain. 100 grains from each harvested plot were assessed for grain infection with Ascochyta, Fusarium and Botrytis fungi genera. Grains were sterilised for 3 min in 1% sodium hypochlorite, washed three times in distilled water and planted on potato dextrose agar ('Merk', Germany). Plates with grains were incubated a the growing chamber at 22°C for 7 days, with 12 h light and 12 h darkness. Species were detected and identified under morphological features according to Mathur and Kongsdal (2003).

Meteorological conditions
Air and soil temperature and amount of precipitation during the growing season (April to August) were obtained from Dotnuva Meteorological Station located about 1.0 km from the experimental site ( Figure 1). The two first growing seasons were similar in the amount of precipitation, with 246.7 and 244.5 mm in 2018 and 2019. The average air temperature was 16.9°C and 15.6°C in 2018 and 2019, respectively. Differently, a higher amount of precipitation (332.9 mm) and lower average air temperature (14.5°C ) during the growing season was in 2020. The average soil temperature at a 5 cm depth was 18.5°C in 2018 and 16.7°C in 2020.

Statistical methods
To analyse the dataset analysis of variance (ANOVA) was conducted. Data were compared using one-way and three-way analysis of variance (ANOVA) and means were separated using least significant difference (LSD) at a 95% probability level. The standard deviation of the seed infection variation on seeds was calculated.
Statistical analysis was made using statistical software SAS 9.4 (SAS Institute Inc., USA).

Results and discussion
The growing of sustainable crops, peas as well, is increasing and an environment-friendly approach to technologies or elements of technologies is needed. With the increase in the area of legume one of the most possible risks is the occurrence or event outbreaks of diseases. An earlier study in Lithuania showed that even at the seedling stage the pea can be heavily affected by a complex of seed and soil-borne pathogens. The authors of this study found that the main pathogens causing pea stem base and root rots were found to be Phoma pinodella and Fusarium genus fungi, among which the most prevalent were F. oxysporum, F. moniliforme and F. avenaceum (Gaurilčikienė et al. 2008). In our research foot rot in commercial cultivars and new pea breeding lines spread from the early growth stage in all experimental years at both infection levels: natural field conditions and in disease-controlled conditions. Foot rot severity depended on year, cultivar or breeding line and growing conditions (Table 1). The presented results from plots with natural field infection show that breeding lines 3784-1 and 3795-3 (Egle DS) were less damaged by foot rots in the early plant growth stage, while Jūra DS, 3637-2 (Lina DS) and 3795-3 were slightly healthier at the end of active vegetation (at BBCH 79-81) than other cultivars. Commercial pea cultivar Ingrid was more sensitive to the foot rots than other cultivars. Early seedling infection was very low in 2018 when favourable soil conditions (wet and warm) for germination prevailed. Moderate seedling blight infection was recorded in 2019 and 2020 in cool and dry weather conditions at germination and early growth stages, respectively. Seed treatment effectively reduced seedling blight infection when the disease index was at a moderate level. Other authors' seed-treatment fungicides are identified as effective measures to manage field pea root rot. However, under a high disease pressure, traditional management practices such as seed treatment fungicides and crop rotation do not provide satisfactory management (Modderman et al. 2018;Gossen et al. 2016). Another study also indicated that seed treatment fungicides reduced the severity of stem base and root rot diseases in field peas (Gaurilčikienė et al. 2008). In our field experiments, foot rot index before the harvest was high for all cultivars and breeding lines. According to our findings, the seed treatment did not show any effect on foot rot severity at late growth stages. The results from our research on seedling blight and foot rot occurrence during the three-year period can be concluded that when favourable soil conditions (wet and warm) for crop germination prevailed, seedling blight infection was low at both infection levels: at natural field infection and under diseases control using seed treatment and foliar fungicides. At cool and dry weather conditions at germination and early growth stages, moderate seedling blight infection was recorded. Seed treatment effectively reduced seedling blight infection when the disease index was at a moderate level. Ascochyta blight is a serious disease of peas worldwide. All above-ground portions at any growth stages of pea plants are susceptible (Skoglund et al. 2011). During our study, except in 2018, Ascochyta blight occurred in all field pea cultivars and breeding lines. The disease infested whole plants and pods as well (Table 2). According to Salam et al. (2011) pre-sowing rainfall significantly affected Ascochyta blight severity. The less disease severity was established when a higher amount of precipitation before sowing occurred.
In our study, similar results were obtained. No visible disease symptoms when a high amount of precipitation in April 2018 were recorded. Meanwhile, dry conditions in 2019 and 2020 spring probably resulted in the occurrence of Ascochyta blight. The pea cultivars showed different susceptibility to Ascochyta blight. Our results are comparable with Olle and Sooväli (2020) where the incidence of Ascochyta blight in field peas is dependent on the cultivar. In the present study, 3795-3 (Egle DS), 3637-2 (Lina DS) and 3784-1 were more resistant to Ascochyta blight on above-ground plants and pods, while commercial cultivars Astronaute, Ingrid and Jūra DS were more sensitive. Fungicide seed and foliar treatment to 67.8% and 12.8% reduced Ascochyta blight severity on plants in 2019 and 2020, respectively. According to other authors, Ascochyta blight severity in peas was significantly reduced by seed treatment and/or by fungicide application (Česnulevičienė et al. 2014).  Note: LSDleast significant differenceat * -P < 0.05 and ** -P < 0.01.

Brauna-Morževska et al. (2019) specified that nonhost-specific
Botrytis cinerea was identified on 586 plant genera including field peas as well. This fungus causes a grey mould on leaves and pods. In our experiment damaged plants and pods by grey mould were more severe in the last experiment year (Table 3)  showed some resistance to grey mould, while Astronaute and Jūra DS were more sensitive to this disease. Jūra DS and 3795-3 (Egle DS) were less damaged by grey mould on pods, while 3637-2 (Lina DS) was more sensitive.
Pea grain yield was significantly affected by cultivar and breeding line, experiments year and growing conditions (2 backgrounds: natural disease infection and diseases under control) (Table 4). New breeding lines produced significantly higher yields than commercial cultivars, while dry weather conditions prevailed during the active pea growth period in 2018 and 2019. Meanwhile, when moisture during the active growing period of peas was sufficient, the new breeding lines and commercial cultivars gave similar grain yields with no differences. It is noted by other authors that a dry season resulted in a higher yield loss of field peas than at the wet season (Salam et al. 2011). Experiments year 2018 was not favourable for disease occurrence in pea; therefore, at low infection level in peas differences between yield at 2 different infection backgrounds was not established in tested cultivars. Meanwhile in 2020 when infection in peas was quite severe, a tangible effect of disease control during the season was established. Based on the results obtained in field trials over  three years all three breeding lines 3637-2, 3784-1 and 3795-3 showed significantly higher yield increases than grown commercial cultivars. Ascochyta complex was identified as dominating pathogenic fungi in field peas harvested grains in our experiments in all tested years, followed by Fusarium spp. and in the lowest amounts Botrytis spp. (Table 5). In our study, from the Fusarium complex, Fusarium poae dominated in damaging harvested pea grains in all experimental years. The pathogenic fungi on harvested grains varied during the experimental year. A higher amount of precipitation during the vegetation period in 2020 (Figure 1) increased pathogenic fungi infection on harvested pea grains. According to Marcinkowska (2008) precipitations increased the frequency of infected seeds, especially in locations with higher precipitation and where lower temperatures dominated. The results of our analyses show a slight tendency that 3795-3 (Egle DS) harvested grains were less damaged by the Ascochyta complex, whereas Ingrid and 3637-2 (Lina DS) were more sensitive to this pathogen.
Didymella pisi was dominated the specie of the Ascochyta complex on harvested grain in all experimental years, while Mycosphaerella pinodes and Phoma pinodella in has higher amounts found in 2020 only (Table  6). Previous studies carried out in two locations in Lithuania showed differences in the prevalence of Ascochyta species on pea grains. D. pisi was more common in the middle of Lithuania (Dotnuva), while M. pinodes and P. pinodella in south part of Lithuania (Perloja) (Česnulevičienė et al. 2014). The infection level of Ascochyta species on pea grains was not high; therefore, the susceptibility of tested breeding lines and cultivars was not reflected during the study period. In Poland also no clear response was established of pea cultivars to Ascochyta blight fungi occurrence on seeds (Marcinkowska 2008). Distribution of these fungi can be caused not only by cultivar but also by weather conditions (Marcinkowska et al. 2009) According to the three-year results found in the present study, a few messages can be presented Seedling blight and foot rot occurred during research years. Dry and cool weather conditions at field peas sowing and germination increased the risk for seedling blight infection and seed treatment with plant protection product was targeted. When at sowing and crop germination wet and warm weather prevailed risk of seedling blight infection was low and seed treatment was insignificant. The footrot index before the harvest was high for all cultivars and breeding lines and no seed treatment efficacy was fixed.
The field peas during the season were damaged by Ascochyta blight at different intensities depending on the year cultivar/breeding line and diseases control efficacy. Ascochyta complex was identified as dominating pathogenic fungus on field peas harvested grains as well and D. pisi specie prevailed.
Pea grain yield was significantly affected by cultivar/ breeding line, experiments year and growing conditions. When dry weather conditions prevailed during the active Table 5. Harvested grains infection in field pea during 2018-2020.
pea growth period, new breeding lines produced a higher grain yield than commercial cultivars. Meanwhile, when moisture during the active growing period of peas was sufficient, the new breeding lines and commercial cultivars gave similar grain yields. This finding indicates higher drought stress tolerance of tested breeding lines. Seasonal challenges obligate more focus on stress-resistant cultivars in the North region as well.
The highest yield difference between growing conditions (natural field infection and under disease control) was recorded in 2020 when Ascochyta blight and Grey mould gave the most severe attack. This finding illustrates the importance of disease control during severe attacks, but for correct control time forecast about infection, risk is important.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This project has received funding from European Regional Development