Resistance Of Litchi Cultivars To Litchi Downy Blight Caused By Peronophythora Litchii

Litchi downy blight caused by Peronophythora litchii is one of the most important litchi diseases.The disease control is mainly depended on chemical spray which may cause resistance and residual problems,etc.However,only a few researches on the resistance of litchi cultivars to litchi downy blight have been reported.The virulence of P.litchii isolates were tested,the inoculation methods of Litchi downy blight were compared,the penetration time of P.litchii was tested,and the resistance of litchi cultivars against litchi downy blight were tested in this study.The main results were as follows:1.The virulent isolates screening:The virulence of 43 isolates of P.litchii was tested.The results proved that the virulence between isolates was significantly different.Among them,the most virulent isolate was 15GDCHr23 with lesion diameter of 14.47±1.47mm at72h post inoculation.Two isolates were avirulent with no lesion on the inoculated fruits.2.The suitable inoculation methods establishing:Comprehensive comparison of four kinds of inoculation methods,both of the spray inoculation and dip inoculation could not distinguished the infection process because these tow types of inoculation methods caused multi-point infection in which the disease incidence and severity developed too quickly and too serious.Even though the mycelial plug inoculation was single point but the disadvantage was that the volume of inoculation could not be accurately quantitative.The drop inoculation was selected as the inoculation method for the determination of disease resistance of different varieties because the infection process was easy to observe due to its single point infection as well as both of the amount and concentration of inoculum suspension were easy to determine.20?l of three concentrations of sporangia suspension were respectively dropped on the fruit surface.The results indicated that the disease incidence increased with the raising of concentration.The disease incidence was 73.3%and the lesion diameter was 13.67±0.11mm at 72h post inoculation while the disease incidence increased to 93.3%and the lesion diameter increased to 17.37±2.12mm at 72h post inoculation when the concentration increased to1×10~4 sporangia per ml,and the disease incidence was even 100%and the lesion diameter was29.10±2.23mm at 72h post inoculation when the concentration was 1×10~5 sporangia per ml.These results proved that the drop inoculation was suitable for resistance tests because both of the concentration and the volume of the sporangia suspension could be quantitative and both of the disease incidence and lesion expansion were increased gradually with time.3.The identification of resistance of litchi cultivars to litchi downy blight:27 litchi cultivars were inoculated respectively with 20?l sporangia suspension(concentration of1×10~4 sporangia per ml)from the virulent isolate of 15GDCHr23 for testing their resistance against litchi downy blight.Both of the disease incidence and severity were investigated at72h post inoculation.The 6 scales of resistance value criterion were established respectively with the threshold of 40%disease incidence and with the threshold of 40 disease index based on the five lesion scales.The results proved that there was no immunity,high resistance,and moderate resistance cultivars,and was only one low resistance,that was cultivar Jingganghongnuo with the disease incidence of 36.7%and the disease index of 28.1 among the 27 tested litchi cultivars.