Onion growers in the Antelope Valley region have been affected by a leaf blight in the past several years. Although a bacterial disease is suspected, the exact cause of the problem has not yet been determined.
Affected plants develop water-soaked lesions that appear first on the outer leaves before spreading onto the younger inner leaves. The elliptical lesions, which spread up and down the leaf blade, eventually become dry and papery as they age. In some fields the majority of the plant canopy may be affected.
In some fields, in the past two years well over half of the canopy collapsed. Although the lesions never spread into the bulbs, yields are apparently reduced since the bulbs are smaller than normal, presumably due to the loss of leaf area.
The malady was more severe in organic fields, but was a significant problem in conventional fields too.
Repeated attempts to isolate a pathogen were unsuccessful last year. A bacterium, tentatively identified as a Xanthomonas species was the primary suspect although Xanthomonas on onions has never been reported in California. The problem again surfaced this year in the Antelope Valley, apparently causing considerable yield loss. Perhaps because plants were sampled at an earlier stage, a bacterium that closely resembles Xanthomonas has been isolated this year.
Growth on a selective medium and colony morphology strongly indicate that it is a Xanthomonas species. More exact methods of identification are in progress along with greenhouse tests. Until these test are complete, we can only tentatively conclude that the cause of the disease in the Antelope Valley is Xanthomonas leaf blight.
The importance of this discovery is that Xanthomonas leaf blight has only been reported in a few places in the world, the first time in Hawaii in 1974. Since then, it has been found in Texas, Colorado, and a few other places in the world. Because it is a relatively new disease of onions, very little is known about it. How it was introduced, spreads, and what can be done to control or manage it needs to be investigated.