Histopathological studies with electron microscopy (SEM & TEM) of Alternaria alternata , causal agent of brown spot and black rot diseases (as postharvest diseases) on citrus in the north of Iran.

摘要

Alternaria species cause two different diseases on citrus in the North of Iran: Alternaria brown spot of tangerines and Alternaria black rot of 'Navel' oranges. Alternaria black rot is also well known as postharvest disease on citrus. In this study infected tissues were collected from leaves, fruits and young stems of the plants. The tissues were cut to 2x2 mm pieces and were fixed over night at 5 degrees C in 0.2 M phosphate buffer including 2% gluteraldehyde. Post fixation was in 1 M osmium tetroxide for 5 h. Specimens were then rinsed and taken through a series of alcohols of increasing concentration and finally lyophilized for 5 h. The specimens were coated with gold and then studied using a ZIESS DSM-960A. For TEM, specimens after being fixed in gluteraldehyde and osmium tetroxide, were embedded in spurr's medium and thin and ultrathin (200-500 nm) (70-120 nm) sections were stained and shown by ziess transmission electron microscope. The present study showed that the court of infection is through stomata and direct. Penetration conidia were produced and not appressoria formation was seen through stomata. Hypha penetrated through the sub-stomata cavity. Some of the hyphal branches were grown in intercellular space of the mesophyll and paranchyme tissue surroundings. The hyphal product, especially toxine (HST and NHST) caused damaged cell and cell wall duration. This study showed that the hypha did not penetrate in xylem tissues. The albedo part of fruit was the main site of penetration and infection and fruit skin, especially in undeveloped fruit, was completely resistance against hypha of the range. Mycelia did not penetrate in the skin and segregate gland. This study has broaded our knowledge of the penetration and control strategies of Alternaria alternata especially on citrus.