Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/1999
Publication Date: N/A
Interpretive Summary: Black rot of cantaluope fruit, caused by Didymella bryoniae, can cause severe field losses with additional losses after harvest. In 1997, a severe epidemic of black rot occurred in the Rio Grande Valley of Texas causing approximately $15,000,000 loss. This study was designed to determine symptoms of black rot on cantaloupe fruit, developmental stage of ffruit when infection occurs, and determine the relationship between fungal virulence and benomyl resistance. The disease can be easily confused with Fusarium fruit rot making field diagnosis difficult if not impossible. Symptoms vary greatly depending on age of fruit when infection occurs. Most fruit infection appears to occur between 12 and 25 days after pollination which is the early stages of net development. Immature fruit (10-day post-pollination) were more susceptible to decay than more mature fruit (20 days or more post-pollination). There was a positive correlation nbetween black rot severity and the cell-wall degrading enzyme polygalacturonase (PG). However, there was no apparent relationship between benomyl resistance in the fungus and its ability to cause disease in the fruit.
Technical Abstract: Black rot of cantaloupe fruit, caused by Didymella bryoniae, can be severe when environmental conditions and fruit developmental stages are favorable for infection. Symptoms of black rot on cantaloupe fruit varied greatly depending on fruit age. The black rot phase was observed only in mature fruit. Inoculation of cantaloupe fruit at different developmental stages with five D. bryoniae isolates demonstrated the greatest amount of decay o 10-day old fruit as compared to 20-, 30-, 40-, or 50-day old fruit. There was no difference in lesion size between 20-, 30-, 40-, or 50-day old fruit, although there was variation between fungal isolates. Each of five fungal isolates produced the greatest polygalacturonase (PG) activity in inoculated 10-day old fruit as compared with that in 20-, 30-, 40- or 50- day old fruit. There was a positive correlation between lesion size and total fungal PG activity in decayed tissue. Using a representative D. bryoniae isolate (OK 963096), multiple PG isozymes were detected in both fungal shake culture and decayed fruit. Eleven PG isozymes (pI 4.7 to 7.9) were detected from fungal shake culture using pectin or polygalacturonic acid as the sole carbon source. Twelve PG isozymes (pI 4.7 to 8.7) were detected from decayed tissue of 10-day old fruit, and 13 PG isozymes (pI 4.2 to 8.7) were observed from decayed tissue of 50-day old fruit. The activity of D. bryoniae PG produced in vitro and in vivo was optimum at pH 5.0 and 5.5, respectively. The activity of the fungal PG produced in vitro exhibited primarily endo-mode of action. In contrast, PG extracted from decayed tissue was predominately exo-PG. The experimental results suggest that PG may play an important role in pathogensis of D. bryoniae during cantaloupe fruit decay.