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EVALUATION OF DOWNY MILDEW (HYALOPERONOSPORA BRASSICAE) INFECTION SEVERITY ON DIFFERENT CRUCIFEROUS OILSEED CROPS

Copyright © 2019 The Authors. Published by Vytautas Magnus University. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. EVALUATION OF DOWNY MILDEW (HYALOPERONOSPORA BRASSICAE) INFECTION SEVERITY ON DIFFERENT CRUCIFEROUS OILSEED CROPS


INTRODUCTION
Cruciferous oil seed crops such as oilseed rape (Brassica napus L.) and turnip rape (Brassica rapa L.) have been important cash crops for farmers in Estonia since the end of 1990s. Since then, the cultivation area has increased tremendously and culminated in 2010 at 100 000 ha that comprised approximately 26% of total cultivation area of cerealdominated crop rotations (Järvan, 2016; Statistics Estonia, 2019). However, the cultivation area has now stabilized at 75 000 ha (16%) (Statistics Estonia, 2019). Growth in extensive monocultures and short rotation has caused serious problems for oilseed rape and turnip rape with several insects like pollen beetle (Brassicogethes aeneus Fabr.) , flea beetles (Phyllotreta spp.) , etc. and several soilborne diseases such as Sclerotinia stem rot (Sclerotinia sclerotiorum (Lib.) de Bary) (Ilumäe et al., 2009) and clubroot (Plasmodiophora brassicae Woronin) (Järvan, 2016), which are difficult to control. Heavy input of synthetic pesticides for disease and insect control puts considerable economic pressure on farmers (Ilumäe, 2013) and causes resistance to widespread pesticides (Brandes & Heimbach, 2018;Kovacs et al., 2015).
The cruciferous pathogen H. brassicae is an obligate oomycete. Although the downy mildew (Hyaloperonospora brassicae (Gaum)) has been considered a minor disease of oilseed rape in Australia ( Van de Wouw et al., 2016), in the Baltic region (Latvia and Lithuania), UK (Hardwick et al., 1991) and elsewhere (Bankina et al., 2012;Petraitiene, 2006), still lately has reported by Mohammed et al. (2017) Coelho & Montiro, 2003). Symptoms appear as white fungal growth that develops on the underside of leaves and yellow/brown irregular lesions that develop on the upper leaf side. Although downy mildew can infect the host plant during growing period, it is still most severe on oilseed rape at the seedling stage when the pathogen can cause seedling death (Oxley & Evans, 2009). Damage is worse in wet conditions at approximately 15°C (Saharam et al., 2018) and in case of infection on pods (Hardwick et al., 1991). Furthermore, under favourable conditions and in intensive cultivation with short crop rotation, downy mildew could become an acute problem in cruciferous oilseed crops in the northern Baltic region.
Different cruciferous crops like winter oilseed rape, white mustard (Sinapis alba L.) and oilseed radish (Raphanus sativus (L.)) have been used quite successfully as green manure and cover crops to supress soilborne pathogens and reduce foliar disease progression (Larkin et al., 2010;Larkin, 2017;Olanya et al., 2014;Runno-Paurson et al., 2014. Use of cover crops could be a promising alternative disease control strategy for environmentally friendly producers. Oilseed radish, rucola (Eruca sativa (Mill)), white mustard and mustard green (brown mustard) (Brassica juncea L.) have been previously reported as effective trap plants for flea beetles, pollen beetles and cabbage seedpod weevil (Ceutorhynchus obstrictus (Marsham) (Veromann et al., 2012;Metspalu et al., 2014;Kovacs et al. 2017). However, the pathogen H. brassicae persists as oospores on infected plant debris in the soil (George, 2014). Thus, it is very important that cover crop plants are not susceptible to downy mildew and pathogen does not accumulate in the soil.
Behind the high diversity of Baltic potato late blight pathogen (Phytophthora infestans) populations is the result of sexual reproduction, which benefits the survival, adaptability and diversity of the pathogen in the climate of North-Eastern Europe (Kiiker et al., 2018(Kiiker et al., , 2019. Furthermore the presence of sexual reproduction changes the epidemiology (Mayton et al., 2000) and increases the adaptability of the pathogen, thus changes the way in which disease control must be approached (Yuen & Andersson 2013). There are reports from higher pathotypic variation on B. oleracea (Coelho et al., 2012). Thus under the changed climate conditions (Sepp et al., 2018), in case of P. infestans, oomycete H. brassicae could happen similarly scenario that through sexual reproduction the genetic diversity and control problems increases in northern Europe.
By now, the knowledge about downy mildew infection severity on oilseed rape, including other cruciferous oilseed crops, has been missing in the northern Baltic region. Therefore, the main aim of this study was to evaluate downy mildew development and severity on spring oilseed rape, and on five alternative cruciferous oil seed crops (B. juncea, B. nigra, S. alba, R. sativus, Eruca sativa) with the purpose to find species that are less susceptible to disease for possible application as cover crop, in trap cropping or seed production. The study was conducted in climatically two different years allowing for characterization of the variability of downy mildew infection severity.

Field experiment
Downy mildew (H. To minimize inter-plot interactions, there was an 1 m wide buffer zone of bare soil around each plot. The whole experimental field was surrounded by a hay meadow. Plots were sown on the 12 th of May in 2010 and 9 th of May in 2011, at 250 seeds per m 2 (seeds obtained from the seed collection of the Estonian University of Life Sciences). The germination of seed was similar for different species in different years, and no differences were noticed in plant density (on average 200 plants per plot) between plots. In both years, the same standard crop management practices were used in all trial plots. The fertilizers and pesticides were not used. The plant growth stage (BBCH) was assessed using the decimal code system of Lancashire et al. (1991).

Weather conditions
In 2010, the air temperature (Eerika Weather Station present at the site) in May and June were similar to the longterm (42-year) average, but in July, the temperature was 4.1°C higher than the long-term average ( Table 1). The rainfall in 2010 was similar to the long-term average in June, but in July, it was lower with only 38.4 mm (70.6 mm for 42-year average). In 2011, the temperature in May was similar to the long-term average, but June was hotter by 1.9°C and July hotter by 2.4°C ( Table 1). The rainfall in 2011 May was similar to the long-term average, but much lower in June and July (Table 1). In the third ten-day period of July, the warm conditions (with temperatures over 30°C for more than 10 days) were suppressive/unfavourable to downy mildew infection.

Downy mildew assessment
Downy mildew occurrence was monitored from June 23 to July 12 2010 (four observations) and from 1 st of July to 22 nd of July 2011 (four observations), at GS 31 -35 to GS 75 -77 (Table 2). Disease assessments were made under natural infection conditions as a percentage of total foliage once each week, until the culmination of the disease. In all species in each of the replications, ten randomly selected plants per plot were assessed for downy mildew infection. Downy mildew infection on test plants was assessed according to the 0 -100% scale (EPPO Bulletin, 2003). The area under the disease progress curve (AUDPC) was calculated from the date of first occurrence of downy mildew until the last observation of the disease in the trial according to Shaner & Finney (1977) by using the following formula: ΣR = n[(Ri +1 + Ri)/2)] (ti+1ti), where Ri is the disease severity (percentage leaf surface blighted) for the previous (ith observation), and Ri+1 the severity for the current observation and ti and ti+1 are the corresponding times of measurements (day of year), and n is the total number of observations.

Data analysis
Statistical analysis of collected data was performed with Statistica 13 (Quest Software Inc, Aliso Viejo, CA, USA). Differences in the severity of downy mildew infection in dependence of year, cruciferous crop species and their interaction were tested by ANOVA. Both factors "year" and "species" were treated as fixed categorical variables. Tukey HSD posthoc tests (P = 0.05) were applied to separate the differences among the means between years and cruciferous crop species. The level of statistical significance for all effects was P = 0.05 except when noted.

RESULTS AND DISCUSSION
In 2010, first occurrence of downy mildew infection was found on the 23 June with slight infection on S. nigra and B. napus plants (F5,12 = 5.85, p < 0.01) (Figure 1a). On the 30 June, the downy mildew infection on B. juncea plants reached 11.7% (average ±SE of replicate plots 0.50%), which was significantly higher than that on other test plant species (F5,12 = 13.52, p < 0.001). At that date, the plants of S. alba were only slightly infected (Figure 1a). Significantly higher downy mildew infection was recorded a week later on the 7 July; at that date, the infection reached 40.4% (±SE 11.0%) on B. nigra and 20.1% (±SE 2.6%) on B. juncea (Figure 1a (Figure 1a).
In this study, we assessed the development of downy mildew on B. napus (the main crop) and compared it with the disease development in five alternative oilseed crops. In both growing seasons, the weather conditions were such that downy mildew assessments were possible, but the development of the disease was somewhat different. June 2010 was wet and cool, thus excellent for infection and spread of downy mildew. In 2010, the disease infection started a week earlier, but the infection pressure was lower, and downy mildew spread stopped also earlier compared to the growing season 2011. Overall, the growing season 2011 turned out more favourable for downy mildew development. However, conidia of H. brassicae can germinate in temperatures ranging widely from 5 to 30°C, and the optimum for germination is 15 − 25°C (Achar, 1998). Downy mildew pathogen conidia germinate when exposed to free water, and once germinated the most rapid development of downy mildew occurs at temperatures between 20 − 24°C (Larka, 2001).
Based on AUDPC values, downy mildew pressure differed significantly between the two observation years (F1,24 = 32.35, P < 0.001) and test species (F5,24 = 50.25, P < 0.001). Although the disease pressure in 2010 was lower, there were still significant differences in AUDPC values between six studied crops (F5,12 = 11.03, P < 0.001). In 2010, two crops, E sativa and R. sativus, had no infection at all, and S. alba had a very low AUDPC value of 14.8 ( Figure 2). In this year, the highest value of 344 was observed in B. nigra. Similarly, in 2011, significant differences between six test crops were found (F5,12 = 9.08, P < 0.001). AUDPC values were very low in three crops: < 2.0 in E. sativa and R. sativus and 8.9 on S. alba (Figure 2). The highest AUDPC values were 751 in B. napus and 608 in B. nigra plants (Figure 2). Combined two-year data showed a significant interaction effect of year and species on the AUDPC value for downy mildew (F5,24 = 19.67, P < 0.001), indicating that B. napus and B. nigra were significantly more susceptible in 2011 than 2010.
Quantitative disease resistance assessment on downy mildew with AUDPC showed significantly lower infection values in the 2010 growing season than in 2011. Downy mildew caused diseases are polycyclic. Each step of the life cycle, including conidial germination, infection, latency and sporulation, is influenced by weather conditions (Kofoet & Fink, 2007). In addition, significant differences in foliar resistance to downy mildew were found between six tested cruciferous crops.   (Mohammed et al., 2019). Given that downy mildew poses a considerable threat to oilseed crops, resistant species and resistant cultivars available for several crops should be harnessed, especially in organic management.
On both growing seasons, the temperature was warmer than long-term average. Our results showed strong interaction between year and species to the downy mildew susceptibility. There are several reports of severe attacks of H. brassicae on cruciferous oilseed crops or vegetables (Mohammed et al., 2017;Coelho et al., 2002;Carlsson et al., 2004). Thus, in more favourable conditions pathogen H. brassicae is able to cause even higher damage as in this study found in northern Baltics. Therefore more resistant crops and cultivars is necessary to seek out and cultivate. * Error bars denote the standard error of the means. Means followed by different capital letters above each bar indicate a significant influence (P < 0.05) of cruciferous oilseed crops in different years (Tukey HSD test). Means followed by different small letters above each bar indicate a significant influence (P < 0.05) of year (Tukey HSD test).

CONCLUSIONS
These results provide useful suggestions in environmentally sustainable plant cultivation (green manure and cover crops) and plant protection (trap crops) strategies, which are based on multitrophic interactions. In both growing seasons downy mildew (H. brassicae) infection was registered on most test crop species. The two growing seasons were different, but the results clearly highlighted a consistent variability in the susceptibility to disease among studied species. Based on our field trial results, S. alba, E. sativa and R. sativus had significantly less downy mildew infection compared to other tested crops. Furthermore, in 2010 downy mildew on E. sativa and R. sativus was not recorded. Besides oilseed rape (B. napus), also black mustard (B. nigra) was susceptible to downy mildew. This must be considered especially in organic cultivation where synthetic fungicides are not allowed for disease control. The results of our study demonstrated that downy mildew is for spring oilseed rape and black mustard in Estonian field conditions a considerable, rather than minor disease. Given that wetter conditions are expected under global climate change in Estonia (Sepp et al., 2018; Climate Change Adaptation Development Plan until 2030), that can particularly favour polycyclic diseases, we argue that downy mildew can become an even more significant pathogen in Estonia in future climates. Thus, more attention should be paid to cultivar resistance of spring oilseed rape and other cruciferous oilseed crops and resistance to downy mildew should be included in local resistance breeding programmes.