Potassium phosphite and ulvan in the control of 'Gala' leaf spot on apple

This work aimed to make a comparative study of the mode of action of potassium phosphite and the polysaccharide ulvan on Colletotrichum gloeosporioides "in vitro" and on apple seedlings. Different concentrations of potassium phosphite (0, 0.0625, 0.125, 0.25 and 0.50 mu L/mL; 40% P(2)O(5) - 20% K(2)O) and ulvan (0, 1, 5, 10 and 20 mg/mL) were evaluated against fungus mycelium growth on PDA-medium. To verify the preventive effect, apple seedlings were grown in greenhouse, their lower leaves sprayed with phosphite (3 mu L/mL) or ulvan (10 mg/mL) and inoculated with a suspension of 3 x 10(5) conidia/mL, 3 or 6 days after treatment. On the 4(th), 6(th) and 8(th) day after inoculation, the percentage of leaf area with necrosis was evaluated on lower as well as upper leaves and the AUDPC was calculated. To verify the curative effect, phosphite or ulvan was applied 24 h after inoculation. Phosphite reduced mycelial growth on culture medium, while ulvan did not affect it. The preventive treatment of ulvan reduced disease severity locally and systemically by 65%. In contrast to ulvan, phosphite presented a curative effect when applied 24 h after inoculation. The results indicate that potassium phosphite has a direct effect on fungus, whereas ulvan probably induces resistance to 'Gala' leaf spot in apple plants.