Part 2: Seed Health Testing & Thresholds

Seed is a potential vector for transporting plant pathogens around the world. Because scientists, growers and regulatory agencies recognize that certain pathogens are particularly aggressive, they have imposed quarantines for some pathogens on certain crops. By preventing pathogen introduction into new geographical areas, quarantines protect seed supplies against seed-borne pathogens. However, most pathogens are microscopic, so in order to establish appropriate quarantines, scientists must use accurate methods to test seeds for diseases.

Seed health testing is an important and growing field. Dr. Lindsey du Toit, associate professor of plant pathology at Washington State University, says the current practice of shipping seed around the world is a major impetus for seed health testing, treatment and development of thresholds. “We don’t want to be moving pathogens around the world,” says du Toit. “It’s not like you can pick up a seed and see that it’s infected. We have to use technical equipment and methods to look for pathogens on seed. We need to be making appropriate decisions based on valid scientific research.”

Radha Ranganathan, of the International Seed Federation (ISF) in Switzerland, works to coordinate the activities of the International Seed Health Initiative (ISHI). ISHI members include pathologists from public labs and official testing bodies. Seed companies formed ISHI in 1994, when they started exchanging information on seed-borne pathogens and developing methods for their control. However, as the International Seed Testing Federation states, seed health tests differ from place to place, so one country may not accept results from a lab in another country.

Ranganathan says the seed industry’s twofold responsibility to promote seed health includes delivering sufficiently healthy seed to growers and respecting international phytosanitary regulations. “The availability of reliable and efficient diagnostic protocols is a prerequisite for supplying healthy seed to growers and plant raisers,” she explains.

Ranganathan says with its current focus on vegetable crops, ISHI brings together more than 50 seed and plant pathologists from most of the big vegetable seed-producing countries. These researchers develop and disseminate information on test protocols for economically important seed-transmitted diseases.

In addition to the 18 methods already created, ISHI is currently working on 15 testing methods for vegetable seed. These methods are in various stages of development.

Du Toit is among the scientists developing international seed testing protocols for various seed-borne vegetable pathogens. For several years, she has focused on creating a protocol for the pathogen Verticillium, which can destroy a spinach crop. Du Toit and her team are working with scientists from California to discern the most appropriate testing method for Verticillium on spinach seed. Their goal is to establish a standardized international procedure for that specific pathogen-crop combination.

Scientists often focus on only one pathogen-crop combination at a time because it is difficult to run one test for multiple pathogens, especially when scientists are looking for viruses, bacteria and fungi that can all be present on the same seed lot. Viruses cannot be cultured in a petri dish, and many bacteria grow on different media than those used to propagate fungi. Therefore, technicians rely on a variety of methods to determine the types and quantity of infections present in a single seed lot.

In addition, developing a test method is complex because it requires a consideration of disease dynamics. Ranganathan explains that the expression of disease depends on an interaction between the infection level in the seed, the susceptibility of the variety, and the environmental conditions in which the seed is grown. “Comprehensive studies that elucidate all these factors are limited and data are usually scattered and inconclusive. The collective experience and active experimentation within ISHI gives rise to recommended test protocols, as well as related sample and sub-sample sizes,” she says.

Once scientists agree on international seed health test protocols for crop-pathogen combinations, they can establish thresholds to reduce the risk of major crop losses (and thus economic losses) to those pathogens. The threshold is the maximum level of infection deemed acceptable in a seed lot. When seed industry researchers determine the level at which a pathogen might lead to crop loss, they use that information to establish internationally accepted thresholds. If even one infected seed among 20,000 could lead to seed transmission and disease in a grower’s crop, scientists give the pathogen-crop combination a threshold of zero. Such a low threshold is typical for pathogens that can spread with rainfall or irrigation and quickly cause severe losses in the crop. Other factors complicating researchers’ attempts to develop thresholds include the diversity of environments around the world in which the crop might grow and variations in susceptibility of crop cultivars to certain seed-borne pathogens.

Although ISHI does not develop thresholds, members meet every nine to 10 months, and crop-specific groups prioritize the crop-pathogens to focus on. Factors that contribute to each group’s decision include buildup of disease pressures, potential of a disease to cause economic damage, technological developments that allow a method to be improved and current lack of a reliable method to detect a pathogen.

Numerous crop-pathogen combinations are high priority this year, including Xanthomonas campestris pv. campestris in cabbage, bacterial fruit blotch in watermelon (caused by Acido vorax avenae subsp. Citnelli) and Clavibacter michiganensis subsp. michiganensis in tomato.

Current diagnostic methods for seed health involve isolating, detecting and identifying pathogens from seeds and confirming the presence of living organisms as pathogenic by inoculating healthy assay plants. Technological developments have generated fast, simple, potentially more sensitive and less expensive methods that provide an alternate and indirect means of checking for the presence of pathogens. The challenge is to prove these new methods are as (or more) reliable, repeatable and sensitive as the currently accepted methods, and to validate their efficacy. If the new methods prove superior, labs will have access to more efficient seed testing methods and growers will have access to higher-quality, healthier seed.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.