IMPACT OF WHITE ROT (SCLEROTIUM CEPIVORUM) ON GARLIC PRODUCTION IN CALIFORNIA

Robert Ehn

CA Onion and Garlic Research Committee. c/o R3 Ag Consulting, LLC. 1508 Tollhouse Rd., Suite D. Clovis, CA 93611

White Rot (Sclerotium cepivorum) is a fungus capable of infecting any species of the genus Allium. The disease has been found in most of the onion and garlic producing regions of the U.S., Canada and Mexico. It has also been reported in most of the major onion and garlic production area of the world.

White Rot was first identified in California in 1939 and is thought to have been transported to the Gilroy area in the early 1950s. It then spread to Monterey county in the mid-50s and finally to Tulelake in 1959. In the San Joaquin Valley, initial infestations are believed to have occurred in the mid 1970s.

White Rot is typically spread by movement of infected soil or plant matter, on equipment, harvest bags and boxes and by humans. Allium sets and transplants may also carry the pathogen. The reproductive structures may be moved short distances by water, animals and blowing soil. As few as 20 infected plants can contaminate an entire field through disking or cultivation. Once the disease is established, yield reductions of 50% to 80% are possible. To date, over 12,000 acres in the San Joaquin Valley of California have been documented as infected with the white rot organism.

White Rot progresses through a disease cycle, initially forming on a host plant. The reproductive structure for White Rot is sclerotia, a uniformly round structure with a black rind surrounding a thick walled mycelium. The sclerotia structure, roughly the size of a poppy seed, can lie dormant on a decayed host plant for weeks to months after its formation. Germination occurs as a plug of mycelium which is stimulated by organic sulfur compounds formed on roots of a growing garlic or onion plant. The mycelium subsequently invades the roots then stem plate or bulb, destroying tissue of the garlic or onion plant. Within days to weeks, the bulb rots and dies. An infestation can affect from several plants to large areas of a field.

Controlling White Rot in garlic and onion fields has the best potential for success when following a comprehensive approach to manage the disease. To date, no single practice has shown to effectively control White Rot, particularly after a disease outbreak has occurred in a field. This presentation provides direction for an integrated pest management approach for management of white rot.

to the 2004 National Allium Research Conference Home Page