Biological Characteristics Of The Pathogen Peltastr Fructicola Causing Sootyu Blotch And Flyspeck

Sooty blotch and flyspeck(SBFS) is a prevalent disease found in global orchard industry and is usually seen in warm and humid areas. The SBFS pathogens propagate on their host surfaces and then display as black dots or smuts without penetration. Among the various fungal pathogens that cause SBFS, Peltaster fructicola is the most common agent of this disease. This study explored the biological characteristics of P. fructicola and revealed many enigmas as follows:1. We found that P. fructicola abundantly sporulated and formed cream yellow yeast-like colonies when growing on artificial media. The anaphase conidia adhere to mycelia followed by declining sporulation, melanin synthesis, thus darken colonies. It resembles black yeast in terms of certain biological characteristics. On artificial media, P. fructicola conidia germinated after they were swollen and formed germ tubes which then developed into mycelia. The mycelia were short and subsequently produced secondary conidia via enteroblastic from sporulating sites on the swollen mother conidia or mycelia. The sporulating sites were elongated as it was continuously producing conidia and the produced conidia kept on swelling, germinating, and sporulating repeatedly. These results indicate that P. fructicola is dominant in reproductive growth against vegetative growth on artificial media and its propagation depends on micro-circulated sporulation.2. P. fructicola showed inconsistent capabilities of utilizing various carbon and nitrogen sources: the utilization rate of glucose, fructose, galactose and sucrose were higher than that of xylose, xylan and rhamnose; furthermore, concentration of the same carbon source also showed effects on its growth and propagation. The higher the glucose concentration was, the faster the colonies grew and the more it sporulated; as for nitrogen source utilization, P. fructicola showcased highest rate of ammonium chloride and the lowest rate of nitrate; addition of moderate amount of nitrogen sources promoted its growth which, however, was impaired by exceeding amount of nitrogen sources. Interestingly, P. fructicola could still grow on water agar or the media without carbon or nitrogen sources indicating robust survival abilities of P. fructicola in nutrient depleted environment.3. On water agar or SNA media which contains no carbon or nitrogen sources, P. fructicola just underwent vegetative growth through mycelial elongation without producing conidial complexes. However, when there was 0.5 g/L glucose or 1g/L KNO3 present in the media, it switched to yeast-like manner of growth. It was found that P. fructicola spreads on Fuji apple by mycelial elongation indicating that vegetative growth via mycelial elongation under nutrient scarce condition is an adaptive feature of P. fructicola to their host surface ecological niches.4. P. fructicola was able to grow in 15 ℃ to 28 with ℃ the optimum growth temperature at 25 ℃. Moreover, it produced more mycelia in low temperature(15 ℃) but was not tolerant to high temperature. On the other hand, it was able to grow on the media with pH from 4 to 6 and the optimum PH for growth was 4-5 indicating its preference of acidic nutrient conditions. It presented as reddish brown colonies on PDA media with tricyclazole which abolished its melanin synthesis indicating that DHN melanin was present in the melanin produced by P. fructicola. It was able to grow on the media containing 0-20% PEG6000 indicating a good capability of resisting drought conditions.5. Through SEM and TEM observations, we also found that P. fructicola could not only degrade and absorb the wax layer on the surface, but also metabolized some cuticles underlying the wax layer without penetrating into the cuticle layer.