Several important pathogens of dry beans, including Pseudomonas syringae pv. phaseolicola (the causal agent of halo blight), Xanthomonas phaseoli (the common blight pathogen), and Colletotrichum lagenarium (the fungus responsible for anthracnose) are seedborne. Recommendations for the control of these diseases, therefore, always include the reduction of seed infection through some kind of "clean seed" program.

The seed for most of the dry bean production in the United States is grown in the semi-arid areas of the Pacific Northwest, where there is very little development of these important seedborne pathogens. In most years the seed produced in these areas has a vanishingly low incidence of seed infection.

In the dry bean producing areas of the central and northeastern US, however, the weather during most summers is at least moderately favorable for the development of epidemics of these diseases. By planting only western-grown seed, dry bean producers in the rest of the country can escape serious infection. Suppose, however, that for reasons of economics and politics the eastern growers decide to establish their own local certified bean seed production program. They know, of course, that they are likely to get some seed infection, but they can afford to invest a bit more in protecting the seed crop with fungicides and bactericides than they can the rest of their beans, and new technologies permit the detection of very low levels of seed infection in their certification program.

Despite the frequent use of the term "disease-free seed", zero infection is impossible, and so in any seed certification program it is necessary to establish an acceptable level of seed infection. Without getting into sampling error and sensitivity of the seed assay, which, of course, are important considerations, we can calculate the maximum allowable seed infection very roughly using our knowledge of the epidemiology of the disease(s) in question.

We begin by working backward from harvest, where we have to decide what level of disease we can allow at the end of the season. This is usually based on economic criteria and yield-loss models, and let us suppose for the sake of this example that we have determined that in the case of halo blight the final incidence of disease allowable is 25% of the plants infected.

We next have to decide which of the epidemiological models to use, and since halo blight clearly is polycyclic, we select the logistic model. Now we have to estimate the apparent infection rate of halo blight under the conditions to which the beans are likely to be exposed. (Ideally we would make several estimates of r, each under different environmental conditions, to calculate the acceptable level of seed infection under the whole range of conditions that we expect to encounter in the field.) This can be done by conducting a series of field trials or by looking up some published disease progress data. (See Estimating Model Parameters: Some Examples.) The rest is simply a matter of plugging in our estimates for r, the final disease incidence, and the length of the season into the simple exponential model and solving for initial disease incidence. (See Practical Uses of Epidemiological Models.)

What becomes painfully obvious in this case is that the maximum allowable level of initial disease incidence is so low that it is not practically achievable by seed selection alone. Our best tactic is to purchase seed produced in semi-arid environments where the level of seed infection is, in fact, exceedingly low. Many eastern bean producers could have saved themselves large sums of money by making these simple calculations.

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