Análisis Correlacional

This fungus is considered the most important pathogen for sunflower in the world and is distributed in all regions, under temperate, tropical or subtropical climates (Gulya et al., 1997).

The losses caused by Sclerotinia sclerotiorum depend on the part of the plant affected by the fungus, which can infect roots, stem or sunflower head. The losses attributed to basal rot depend on plant age at the infection. S. sclerotiorum quickly kills the infected plants at seedling stage, resulting in stand failures. Losses associated with head rot directly affect yield, reducing the number of seeds per chapter, seed weight and oil content. The oil quality is lower due to the increased concentration of free fatty acids on seeds infected by the fungus.

Seeds of infected head can fall, resulting in total loss of production (Zimmer & Hoes, 1978;

Davet et al., 1991; Masirevic & Gulya, 1992; Pereyra & Escande, 1994). In the state of Paraná, Brazil, incidence of stem and head disease was high (17.6% to 100.0%), when sunflower was cultivated after the harvest of summer crops in 1996-1998 growing seasons (Leite et al., 2000). Indirect losses occur due to contamination of seeds with sclerotia, often of the same size, shape and specific weight of these, being impossible their removal during

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cleaning operation. In addition to these losses, the fungus persists in the soil for many years, representing a potential danger for sunflower and other hosts (Zimmer & Hoes, 1978; Davet et al., 1991; Masirevic & Gulya, 1992; Pereyra & Escande, 1994).

S. sclerotiorum can cause different symptoms in sunflower (Zimmer & Hoes, 1978;

Almeida et al., 1981; Davet et al., 1991; Masirevic & Gulya, 1992; Pereyra & Escande, 1994;

Gulya et al., 1997).

The basal rot may occur from the seedling stage to maturity. In seedlings, infection is less frequent, as plants die quickly and the process does not result in spread to other plants.

Infection is mostly observed near flowering. Diseased plants appear as a group of two or more plants on the line. Rot starts when the mycelium of the fungus, originating from sclerotia in the soil, comes in contact with the lateral roots. The first symptom observed is a sudden wilting of the plant. The infected plant can recover turgidity at night or after a rainfall, but within a few days, this symptom becomes irreversible, and the disease is named Sclerotinia wilt. A light brown and soft injury appears at ground level and typically surrounds the rod. If there is high humidity, the lesion may be covered by white mycelium. The fungus develops internally and destroys the internal tissues of the stem. Many sclerotia are found within the colonized portion of the stem, but few are found in the root and in the outdoor area. Diseased plants can lodge easily.

The rot in the middle portion of the stem occurs in plants from the end of the vegetative stage to maturity. Infection occurs on leaves wounds and proceeds toward the petiole, ending the stem. The appearance of the lesion is similar to those of the basal rot. It is most noticeable in mature stems, because the affected tissue appears lighter than the normal brown coloring physiological maturity. A white mycelium can cover the lesion, and sclerotia are observed within the stem. Plants can break at the lesion site.

Figure 2. Sclerotinia head rot in sunflower.

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Symptoms in sunflower heads occur at the end of flowering or later. The infection may start in any part of the receptacle. The initial symptoms are characterized by light brown and soft lesions on the dorsal side of the head covered by portions of white mycelium (Figure 2).

Eventually, the fungus destroys the interior of the head, leaving intact only the vascular elements. Sclerotia in large numbers and irregular shape are found within the head. Finally, there is the complete disintegration of the head, which remains with the exposed fibrous vascular elements, like a broom. A mass of achenes and sclerotia falls into the base of the plant.

Sclerotinia sclerotiorum (syn. Sclerotinia libertiana Fuckel and Whetzelinia sclerotiorum (Lib.) Korf & Dumont) form mycelium and sclerotia in the asexual phase and asci with ascospores in the sexual phase. Microconidia are produced in senescent cultures in the laboratory, can be functional for asexual reproduction, but their role in the biology of the pathogen is not known. The mycelium is composed of hyaline multicellular hyphae with 6.5 to 7 µm in diameter (Mordue & Holliday, 1976).

Sclerotia is formed from the anastomosis of a large number of hyphae in a hard body with variable format, and may reach several centimeters long. Sclerotium germination occurs in two ways: a myceliogenic way forming only hyphae and another carpogenic way producing apothecia (Masirevic & Gulya, 1992; Gulya et al., 1997).

The apothecium is a flat or cup-shaped structure that produces sexual spores of S.

sclerotiorum. Many apothecia may be formed from a single sclerotium. The apothecia are pale brown in color and have 4 to 10 mm in diameter. Moist soil for a long period and light are essential for the formation of apothecia. The upper layer contains many paraphyses and asci. The asci are cylindrical and extended to the apex and range 130-163 µm long and 8 to 10 µm wide (Mordue & Holliday, 1976; Zimmer & Hoes, 1978; Davet et al., 1991; Masirevic &

Gulya, 1992; Gulya et al., 1997).

S. sclerotiorum is a polyphagous fungus, having as host plants of 75 families, 278 genera, 408 species and 42 subspecies or varieties. Except one species of the phylum Pteridophyta, all hosts of S. sclerotiorum belong to phyla Gymnospermae and Angiospermae (Boland & Hall, 1994). No reports of physiological specialization of the fungus are available (Mordue &

Holliday, 1976; Gulya et al., 1997).

The sclerotium begins and ends the life cycle of S. sclerotiorum (Zimmer & Hoes, 1978).

Miceliogenic germination of sclerotia causes infection on tissues of the base of the plant to produce root rot, stem rot and wilting (Davet et al., 1991). The hyphae penetrate through tissue injuries or stomata by the cuticle, invading the intercellular spaces, and finally reach the interior of cells. The fungus causes lesions visible at the base of the stem and wilting of shoots due to obstruction of the conducting vessels (Pereyra & Escande, 1994). Secondary contamination is possible through direct contact of the diseased tissue with healthy tissue from neighboring plants (Davet et al., 1991). Carpogenic germination of sclerotia generates the apothecia, which emerge on the soil surface and release the ascospores. In conditions of high relative humidity above 70% a mature apothecium can produce up to 2 x 10⁸ ascospores over a period of several weeks. The ascospores are released at temperatures of 3ºC to 22ºC, with greater intensity between 19ºC and 20ºC. Temperatures above 25ºC and relative humidity below 35% are limiting for ascospore survival. The ascospores germinate in favorable conditions and infect the host, causing mainly head and stem rot. The susceptibility to infection of the sunflower head is higher in the period between the initial flowering and up to two weeks after flowering. After a latent period of 15 to 40 days, the fungus invades the

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parenchyma and causes tissue rotting. The optimum temperature for development of the mycelium is between 18ºC and 25ºC. The sclerotia produced within and on the surface of the colonized tissuse return to the soil and crop debris are responsible for the conservation of the fungus (Zimmer & Hoes, 1978; Davet et al., 1991; Gulya et al., 1997). The sclerotia can remain in the soil for many years, keeping intact their pathogenic power (Pereyra & Escande, 1994). Seeds are important vehicles for the dissemination of S. sclerotiorum, as sclerotia mixed with the seeds or fungus mycelium colonizing the internal tissues (Mordue & Holliday, 1976; Zimmer & Hoes, 1978).

Sclerotinia wilt and head rot control is difficult due to the persistence of sclerotia viable for a long time in the soil, to the fact that the fungus produces ascospores responsible for fungus through contaminated seed from other countries. An ordinance of the Brazilian Ministry of Agriculture, Livestock and Supply established the importation of seeds only from sunflower production areas free of S. sclerotiorum.

Genetic resistance to Sclerotinia wilt and head rot has been studied in several countries.

Efforts have been made in breeding programs around the world aimed at finding resistance to the pathogen, but little progress has been made (Zimmer & Hoes, 1978; Gulya et al., 1997).

All studies indicate a lack of immunity in the sunflower cultivated and other wild species, similar to what is observed in all species of plants are affected by S. sclerotiorum (Gulya et al., 1997). The resistance of the sunflower to S. sclerotiorum is partial and controlled by multiple genes. The reaction of the same genotype may vary depending on the mode of fungus attack, or one genotype can display a high level of resistance to basal rot and show to be very sensitive to head rot. Furthermore, genes that are expressed in a stage of plant development may be ineffective in another stage (Davet et al., 1991). Wild species of Helianthus, such as H. resinosus, H. debilis, H. lenticularis and H. petiolaris, have high resistance genes (Zimmer & Hoes, 1978; Davet et al., 1991). There are reports of variation among cultivars for incidence of head rot, but apparently these differences are related to greater plant height, which would provide less favorable conditions for fungus infection (Zimmer & Hoes, 1978). Restorers and maintainers oilseed sunflower germplasms with improved resistance to head rot have already been developed and released (Miller & Gulya, 2006; Miller et al., 2006), but until now there are no hybrids or commercial varieties that have resistance level suitable for cultivation of sunflower in areas where this disease is endemic.

Methods including mapping of Quantitative Trait Locus (QTL), as they detect relationships between phenotypic variation and gene polymorphisms in existing germplasm, can be useful in dissecting complex traits in sunflower, like resistance to Sclerotinia diseases, thus providing a valuable tool to assist in crop breeding (Fusari et al., 2012).

Crop rotation is a suitable practice to help management of S. sclerotiorum. The use of crops resistant to this fungus, like grasses, serves to allow time for natural degradation of sclerotia by their natural enemies. Due to susceptibility to S. sclerotiorum, sunflower cultivation after soybean, canola, peas, beans, alfalfa, tobacco, tomatoes and potatoes, among other crops, should be avoided. Rotation with non-host crops during three to five years reduces the number of sclerotia in the soil and minimizes the impact of the sunflower root

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infection (Gulya et al., 1997). Weeds should be well controlled, because they may be alternative hosts for S. sclerotiorum. An obvious recommendation, but very important, is to avoid the use of seed mixed with sclerotia (Pereyra & Escande, 1994).

A key measure to prevent the occurrence of Sclerotinia wilt and head rot is the choice of sowing date, in order to reduce periods of high humidity and low temperature during the cycle, especially at flowering. In southern Brazil, for cultivation of sunflower after the harvest of summer crops, sowing should be performed until mid-March and early-maturity genotypes (100 days between emergence and harvest) should be used, considering studies of zoning for climate risk, to prevent low temperature at the end of the cycle (Leite et al., 2000).

Other cultural practices are important to minimize the problems caused by S.

sclerotiorum. Spatial isolation is a measure effective in reducing the incidence of infection by ascospores. Generally, it is recommended to choose areas at least 1 km away from crops infected with S. sclerotiorum in the previous year (Masirevic & Gulya, 1992). It is convenient to choose lower seeding densities and larger spacing so as to enable adequate plant aeration and decrease the chances of contact with diseased adjacent plants (Zimmer & Hoes, 1978).

Chemical control has not proven to be effective for several reasons. For sunflower, there are no products with systemic efficiency (Davet et al., 1991). The duration of flowering and consequently the susceptibility to head infection requires at least two or three preventive sprays with fungicides. Furthermore, the penetration of products in floral organs is very difficult (Davet et al., 1991) and the fungicide needs to be applied to the head face to be effective (Masirevic & Gulya, 1992). In Brazil, fluazinan is registered for control Sclerotinia head rot in sunflower (Ministério, 2012), but there are no available data that indicates efficiency on disease control until now.