Bacterial fruit blotch
| Bacterial fruit blotch | |
|---|---|
| Common names | BFB |
| Causal agents | Acidovorax citrulli |
| Hosts | Cucurbitaceae (melon and watermelon) |
| EPPO code | PSDMAC |
Bacterial fruit blotch (BFB) affects cucurbit plants around the world and can be a serious threat to farmers because it spreads through contaminated seed. BFB is the result of an infection by Gram-negative Acidovorax citrulli bacteria, which has only been recently studied in detail. Members of A. citrulli are Gram-negative rod shaped bacteria with the dimensions 0.5× 1.7 μm. They move via polar flagella. No known reliable sources of BFB resistance exist today, so seed hygiene and thorough testing of breeding facilities are the best way to control spreading. No known control methods, however, are extremely reliable for reducing BFB infection.
A. citrulli causes disease in the Cucurbitaceae family, with the most significant losses in melon and watermelon. It also affects pumpkin, zucchini and cucumber but these are not as economically devastated by fruit blotch as the melons.A. citrulli’s economic hosts are cucurbits, but the bacteria can also infect volunteer seedlings of other families. This makes it easier for the pathogen to spread. Symptoms of melon with BFB include water soaked lesions on cotyledons, and hypocotyls, leading to collapse and death. Lesions will look necrotic and may be near veins. On fruit, water soaked lesions will be small and irregular (they average 1 cm diameter and may be sunken) but then progress through the rind. The fruit then decays and cracks when the pathogen causes necrosis. These lesions open the plant to secondary infections as well. A. citrulli then colonizes the pulp, eventually allowing the seed to become contaminated. On adult leaves, the symptoms appear the same as the ones left by other abiotic or biotic stressors so diagnosis is not as straight forward. They include large irregular leaf lesions which are brown to black in watermelon and reddish brown in melon. Bacterial fruit blotch lesions spread along main midrib in adult leaves.
Acidivorax citrulli is primarily seed transmitted. Seeds containing A. citrulli are difficult to treat, as the bacteria is found deep within the tissue, and can be viable for 35 years or more. There are still some aspects of the epidemiology that are unknown. It has been found that the pathogen initially starts its life as a saprophyte, relying on the seed's degrading action of complex sugars, and switches to a pathogenic growth mode when the seedling emerges. While this is a step forward, there is little knowledge of the pathogen's movement in the plant, how it overwinters, or its alternate hosts. Without this knowledge, its complete mode of reproduction (an important tool in reducing epidemic) is still unknown.
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