Initially Colletotrichum gloeosporioides was the only causal agent of anthracnose identified in onion. However, another causal agent called Gibberella moniliformis was discovered.
|anthracnose showing twisting symptoms|
This is the first report of two pathogens attacking onions at the same time said Dr. Ronaldo T. Alberto of the Central Luzon State University (CLSU) and Dr. Vermando M. Aquino of the UP National Institute of Molecular Biology and Biotechnology (UP-NIMBB) during the presentation of their research under a research fellowship in agriculture and fisheries supported by the Department of Agriculture-Bureau of Agricultural Research (DA-BAR) and University of the Philippines-Natural Sciences Research Institute (UP-NSRI).
The study titled “Molecular characterization of Colletotrichum gloeosporioides and Gibberella moniliformis (anthracnose/twister disease) infecting onions in the Philippines” aims to characterize and identify the species of Colletotrichum causing the severe form of anthracnose in onion and to determine the sensitivity of anthracnose/twister pathogens to selected commercial fungicides.
|anthracnose symptom in onion leaves|
Anthracnose/twister is a disease that causes white, oval, sunken spots on the leaves of onion at early vegetative stage. It also causes twisting and discoloration of the leaves resulting to elongated neck and slender bulbs. In severely infected onions, dark zonated fungal structures and dieback symptoms appear leading to the collapse of the plants. In the past years, anthracnose seriously damage onions in Nueva Ecija (the biggest onion producer in the country) and neighboring onion-growing provinces in Luzon. The disease causes 80-100 percent yield loss resulting to high price and shortages in supply of onion in the market.
Given such scenario, the disease now poses a serious threat to the most indispensable culinary ingredient in the world and the most profitable single vegetable species in the Philippines after asparagus and tomato. In fact, onion now ranks as the third among vegetables in area of production, fifth in both quantity and returns, and considered as one of the country’s top foreign exchange earners. However, in spite of the increased in areas planted, maximum production was not attained. This is due to the proliferation of insect pests and diseases that continue to strain onion in almost all production areas in the country.
|infected onion bulb|
Currently, there is no effective management yet discovered to control the disease. As the Gibberella moniliformis is a facultative endophyte (living inside the plant) which is hard to control by fungicide application. Nevertheless, formulation of fungicide or other alternative management to control the disease might not be successful without a clear understanding of the nature of the disease. According to Dr Alberto, different strains of the pathogen have different sensitivity response to different fungicides and these differences in sensitivity response can cause a shift in pathogen’s population structure.
In their study, infected onions were collected from different growing areas in Nueva Ecija, Pangasinan, Nueva Viscaya, Ilocos Sur and General Santos City. Colletotrichum species were isolated including the unidentified fungal species derived from leaves, neck and bulb of onions. These isolates were tested for its pathogenicity, morphologically analyzed and compared. Molecular tools such as DNA extraction, amplification, sequencing and phylogenetic analysis were also conducted.
|non formation of bulb and elongated neck cause by anthracnose|
Based on the comparison of DNA sequence to GenBank (an open access sequence database produced by National Centers of Biotechnology Information), the unidentified fungal species show high similarity with Gibberella moniliformis (synonym: Gibberella fujikoroi) – the causal organisms of bakanae disease of rice causing abnormally long and weak internodes and ear and stalk rot to maize. Gibberella moniliformis causes the elongation and twisting symptoms of onions while Colletotrichum gloeosporioides is responsible for anthracnose symptoms.
In addition, pathogens’ sensitivity to fungicides were also determined.
Dr Alberto said, the result of this study is very useful to scientists, extension workers and pesticide industry to address the disease. He further emphasized that thorough knowledge of the pathogens biology, identity and method of infecting their host is the very best weapon is coming up with effective solution and suitable management strategy to neutralize or defeat anthracnose disease that causes low production in onion industry in the Philippines.