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California’s “Clean Seed” Sweetpotato Program

The Issue

Sweetpotatoes are vegetatively propagated. Roots are sprouted and the sprouts are transplanted to the field to produce more roots. True seeds are not used in commercial production because sweetpotatoes rarely flower. An unfortunate consequence of not using true seed, however, is that viruses can accumulate in the plants, greatly diminishing both yield and quality.

What Has ANR Done?

California’s “clean” sweetpotato seed program began in the 1960s in response to russet crack disease, which is caused by a strain of sweetpotato feathery mottle virus. At the time, it was well known in the industry that new seedstock was required to prevent this disease as well as “variety decline,” the gradual loss of yield that occurred in many varieties. To address this problem, in 1961 Farm Advisor Bob Scheuerman and Extension Specialist Dennis Hall began testing a process called meristem culture on sweetpotatoes.

The procedure developed then is still in use today to provide growers with high quality seedstock. It involves aseptically removing the meristem (usually 0.5 mm long) from an apical or lateral bud of shoots produced in a greenhouse from a sprouted root. The meristems are placed in test tubes and grown on synthetic nutrient agar to produce a new plant. After three to four months in culture, the plant is transplanted in the greenhouse and grown out for virus testing. To determine if a meristem-generated plant is free of virus, it is grafted onto an indicator plant (Brazilian morning glory). If that plant shows no disease symptoms, then the sweetpotato plant is assumed to be virus-free, or “clean.” At this point, it can be propagated through cuttings and grown to produce roots for variety evaluation.

By 1966, enough plant material had been developed using this process that Scheuerman and Hall could conduct large scale field tests. The results showed a significant yield increase from virus-tested plants, compared to those that had not gone through meristem culture. Furthermore, there was a substantial decrease in the number of cull potatoes caused by viruses.

The Payoff

Program accepted by industry, copied in other states.

Since these early tests, the benefits of virus-tested material has been well recognized in California and other states. Beginning in the mid 1990’s, Louisiana and North Carolina dropped their traditional seed programs and began producing virus-tested material in greenhouses for their farmers, based on the clean seed program developed by UCCE.

Most growers in California now use virus-tested seed for at least part of their production. The process is performed by Foundation Plant Materials Service (FPMS) on the University of California Davis campus, which provides plants to growers for a minimal fee of $1.00 per plant. The use of virus-tested seed is one of the reasons for the substantial sweetpotato yield increases in the last 30 years. In 1967, average yields were 5 tons per acre; in 2001, 12 tons.

UCCE continues to test and promote the sweetpotato clean seed program. Trials conducted in Merced County in 1999 – 2001 showed improved yield, shape, and color in five commercial varieties from using virus-tested plants. The use and acceptance by the industry in California as well as other states demonstrates how UCCE research and extension programs can have long standing positive impact for the industry. Consumers throughout California and the West have also benefited, as the clean seed program gives them a more consistent, high quality potato that is significantly less likely to have internal defects.


Supporting Unit: Merced County

Scott Stoddard, Farm Advisor, Merced & Madera Counties, University of California Cooperative Extension, 2145 Wardrobe Ave, Merced, CA 95340
(209) 385-7403, csstoddard@ucdavis.edu