Sclerotinia Wilt Occurrence on Sunflower in Vojvodina , Serbia

Sclerotinia sclerotiorim is a facultative parasite that can cause diseases on more than 400 plant species. Sclerotinia wilt presence was observed in Vojvodina during the period 2007-2009 at seven sites (Bečej, Kikinda, Kula, Pančevo, Sombor, Vršac and Zrenjanin). Assessment was done on 8 hybrids (Baća, Bačvanin, Branko, Duško, NS-H-111, Sremac, Šumadinac and Velja). Disease intensity was calculated according to McKinney’s formula. AMMI analysis was used in order to estimate stability of hybrid tolerance to Sclerotinia wilt in different environments. Average Sclerotinia wilt occurrence on sunflower was the lowest in 2007 and the highest in 2009. Sclerotinia wilt was not observed in Zrenjanin and Vršac in 2007, nor in Pančevo in 2008. The lowest average Sclerotinia wilt occurrence during 3-year period was in Sombor and Pančevo. Hybrids Duško, NS-H-111, Sremac and Velja can be recommended for future growing on sites where Sclerotinia wilt of sunflower occurs.


Introduction
Sclerotinia sclerotiorim (Lib.) de Bary is a facultative parasite that can cause diseases on more than 400 plant species, including a large number of important crops such as sunflower, soybean, oilseed rape, edible dry bean, chickpea, peanut, dry pea, lentils and various vegetables (Bolton et al. 2006).Occurrence of diseases caused by S. sclerotiorum on sunflower is influenced by genotype and weather conditions.This fungus favours moist and cold weather conditions prevailing in temperate climate regions where the sunflower is frequently cultivated.Symptoms of diseases caused by S. sclerotiorum can appear during the whole sunflower growing season.This pathogen can cause three different forms of the sunflower disease -Sclerotinia wilt, middle stalk rot and head rot.The infection comes from the infected overwintered plant debris and overwintered sclerotia in the soil or from infected seed.After the infection, fungus spreads and develops fast in the infected tissue.The pathogen has the ability to produce proteolytic enzymes which cause degradation and rotting of the infected tissue.Infected parts of the plant are covered with white mycelium and black irregular shaped structures for overwinteringsclerotia.Sclerotia may conserve their viability in the soil for more than 10 years (Aćimović 1998).Yield loss depends on the sunflower development stage in which the disease occurs.If infection occurs in early sunflower development stage, the yield loss will be approximately equal to the disease occurrence percentage.Disease occurrence percentage and yield losses caused by sclerotinia wilt can even reach 100% because it causes whole plant devastation (Aćimović 1998, Lamey et al. 2000).Sunflower plants infected at the beginning of flowering stage can lose up to 98% of their potential yield, while plants infected eight weeks after flowering can lose only 12% of their potential yield (Maširević & Gulya 1992).
In the USA, annual losses on all crops caused by S. sclerotiorum exceed $ 200 million, while in 1999 Sclerotinia head rot epidemic on sunflower caused crop loss valued at $100 million (Bolton et al. 2006).In Serbia, Sclerotinia wilt is the most common form of Sclerotinia disease, and appears in sunflower crop more frequently than other two forms.Its average frequency in Serbia is about 15-20%, but in some years the frequency can reach even around 50% (Marić et al. 1988).
As Sclerotinia wilt causes economically important yield loss in Vojvodina, Serbia, tolerance of NS commercial hybrids to this disease is regularly estimated under the commercial growing conditions.Sclerotinia wilt occurrence and intensity in at seven sites Vojvodina and tolerance of eight commercial NS hybrids during three-year period were observed in this paper by using McKinney's formula and AMMI analysis.

Materials and Methods
Sunflower natural infestation by S. sclerotiorum and Sclerotinia wilt occurrence under field conditions were observed for three years (2007)(2008)(2009) in Vojvodina at seven sites (Bečej, Kikinda, Kula, Pančevo, Sombor, Vršac and Zrenjanin).These sites were selected as representative for different sunflower growing environments in Vojvodina.Eight NS commercial hybrids usually grown in Vojvodina were included in the assessment (Baća,Bačvanin,Branko,Duško,Sremac,Šumadinac and Velja).Growing conditions for these hybrids were the same as the conditions in the usual commercial cultivation -row spacing was 70 cm while spacing between plants was 25 cm.Eight rows of each hybrid were sown next to each other at all sites.Crop rotation was taken into account at every location, and sunflower had not been grown for at least three years on the observed fields.
Disease assessment was done during R9 growth stage of plant maturity (Schneiter & Miller 1981) by visual observation of typical water-soaked lesions symptoms, white mycelium and sclerotia in the stem pith.Estimation was done on 300 plants per hybrid per site.In order to avoid border row effect, plants from border rows and plants growing within 10 meters from the field border were not included in the disease assessment.
Degree of plants susceptibility to disease was registered according to 1-5 scale (Aćimović 1979): 1-mycelium at soil surface, 2-ring-like necrosis above ground, 3-stem base necrosis, 4-¾ of stem covered with ring-like necrosis, 5-completely wilt plant.Sclerotinia wilt intensity was calculated according to McKinney's formula, and average disease occurrence was calculated as a number of diseased plants in a total number of estimated plants.
The obtained results were also analysed using AMMI analysis in order to estimate stability of hybrid tolerance to Sclerotinia wilt in different environments (Abamu et al. 1998).

Results and Discussion
According to the obtained results, the highest Sclerotinia wilt occurrence was observed in 2007 in Kikinda, while it was not registered in Zrenjanin nor in Vršac (Tab.1).Low occurrence of Sclerotinia wilt was observed in 2007 in Sombor and Pančevo (0.05% and 0.09%, respectively).This was probably caused by unfavourable agroecological conditions for the wilt development -spring of 2007 was extremely hot and June saw total amount of rainfall much lower than average.During summer, average monthly temperatures were higher than long-term average by 2.4°C -5.7°C, while precipitation was lower (Hydrometeorological Institute of Serbia 2008).
In 2008, Sclerotinia wilt occurrence was not registered in Pančevo, while its highest occurrence was in Kula (Tab.1).Low average disease occurrence was registered in Sombor and Bečej (0.21% and 0.29%, respectively).Dry conditions were prevalent during growing season of 2008 with two extreme drought periods -from May 26th till June 3rd and from 20th till 27th of June (Hydrometeorological Institute of Serbia 2009).These were the periods when sunflower plants were most susceptible to disease development (R3 and R4 growth stage), which can explain the lower Sclerotinia wilt occurrence during 2008.According to estimations of 2,400 plants of eight hybrids per site, Sclerotinia wilt average occurrence was the lowest in 2007 (0.55%), middle in 2008 (1.22%), and the highest in 2009 (3.05%).Sclerotinia wilt average occurrence for the three-year period was 1.61% (Tab.1).Similar occurrences of Sclerotinia wilt were registered in the neighbouring Croatia with similar agroecological conditions, where variant treated with fungicide had 2.5-7.5% diseased plants, while in untreated control it varied from 0-10% (Ćosić et al. 2005) According to AMMI analysis, the genotype NS-H-111 had the highest number of healthy plants in Pančevo and at sites with disease occurrence higher than average -Kikinda and Kula (Tab.3).In Sombor and Zrenjanin, Šumadinac was the genotype with the highest number of healthy plants, while the same were Velja in Bečej and Baća in Vršac.Velja had the highest average number of healthy plants at all seven sites in three years (Tab.3).
The main effects of the AMMI analysis, genotype and environment, are 65.75% of total trial variation, of which 43.34% belongs to environment and 22.41% to genotype.Non-additive source of variation, interaction between genotype and environment, showed high statistic significance and accounted for 34.24% in square sum of a trial.First PCA axis makes the most of total variability with 46.81% of interaction, while second PCA axis makes 39.57% of interaction and both have high statistic significance.
Based on AMMI biplot GxE interaction, it can be observed that genotypes with a higher -evoj formuli i izražen u procentu number of healthy plants than average were Duško, NS-H-111, Sremac and Velja.Duško showed the highest stability in its tolerance to Sclerotinia wilt which makes the most suitable hybrid for cultivation in the sites where Sclerotinia inoculum is already present in soil (Fig. 1).Higher than average, disease occurrence was registered in Kula and Kikinda, while the highest variation in a disease occurrence during three years period was observed in Kula and Zrenjanin (Fig. 1).
According to the obtained results, we can conclude that the main cause for Sclerotinia wilt low occurrence in Vojvodina during the estimated period (2007)(2008)(2009) was unfavourable weather conditions for pathogen and disease development.Additionally, high tolerance of NS hybrids to Sclerotinia wilt also proved contributing.Duško showed the highest stability in its tolerance to Sclerotinia wilt, which makes it advantageous for future growing in Vojvodina, especially on sites with higher disease occurrence.Hybrid NS-H-111 is a well known sunflower hybrid that once again confirmed its tolerance to Sclerotinia wilt.Evaluated NS sunflower hybrids during the years of estimation also achieved high average yields (3.14 to 3.51 t ha -1 ) (Miklič 2008, Miklič 2009).

Table 1 .
Sclerotinia wilt occurrence on 8 sunflower hybrids at seven sites inVojvodina (2007Vojvodina ( -2009)  )Regarding growing season of 2009, Sclerotinia wilt occurrence was lowest in Sombor and Pančevo like in 2007.The highest average occurrence of Sclerotinia wilt was in Kula (Tab.1).Similarly to 2008, high temperatures characterized the summer of 2009 and caused low Sclerotinia wilt occurrence.However, unlike previous years, daily rainfall in June and total amount of summer rainfall were above average (Hydrometeorological Institute of Serbia 2010), which caused somewhat higher Sclerotinia wilt occurrence in 2009 than in the previous two years.

Table 3 .
Means of healthy plants of eight sunflower hybrids at seven sites and values of the first two variation sources Tabela 3. Srednje vrednosti zdravih biljaka osam hibrida u sedam lokaliteta i vrednosti prva dva izvora varijacije