Histopathology of Sclerotinia Sclerotiorum Attack on Flower Parts of Helianthus Annuus heads in Tolerant and Susceptible Varieties

Sunflower head rot is a major disease caused by Sclerotinia sclerotiorum .Sunflower varieties which are tolerant to the fungus have been developed. Thechanges occurring in flower parts at different times after inoculation with pathogenascospores were studied for two sunflower varieties (tolerant HA 302 and susceptibleHA 89). In variety HA 302 there was cell collapse, changes in cell wall composition,and an increase in phenolic compounds in the tissues of corolla and style, whichprevented the pathogen from advancing. This response was weaker in susceptiblevariety HA 89, and occurred only in the style, so did not stop the pathogen fromdeveloping and reaching the ovary. Phenolic compounds were found in HA 302corolla and style tissues only when the pathogen was present, constituted an inducedresponse that prevented further development of the fungus. Principal componentanalysis (PCA) showed that at the beginning of the infection there was no differencein behavior between the two varieties. The difference arose during the final observation times, when in variety HA 89, the pathogen colonized ovary, style and base of filaments and produced noticeable colonization of the corolla.