Infection Process of Botryosphaeria dothidea in Apple Fruit

The penetration process of Botryosphaeria dothidea through the lenticels of apple fruit was observed with light microscope and scanning electronic microscope. Condidiospore germination and appressorium formation were completed 2 days after inoculation, the mycelium grew slowly on the lenticel surface 10 days after inoculation. From 20 to 30 days after inoculation, mycelium development speeded up with branching, At this stage, pathogen hyphae only penetrated through the surface tissue of the lenticels, and infection usually occurred around the outer area of the lenticels. From 40 to 50 days after inoculation, the mycelium developed more lushly and some hyphae intruded into the second layer of the lenticels. Low polygalacturonase (PG) activity was detected in inoculated lenticels. The lenticels were composed of three layers. The first layer was dead tissue rich in pectins which were easily degraded during pathogen infection. The third layer at the bottom of the lenticels was very firm and considered as an effective barrier against pathogen penetration. The number, size and cracks of the lenticels increased regularly with fruit development. Our result further demonstrated the character of latent infection in apple ring rot disease.