Creating Disease-Resistant Bedding Plants

Growers face many challenges producing bedding plants — timing, insects, profitability and, of course, disease. Diseases can be particularly frustrating as they often appear just as the crop is ready for sale. For the homeowner, diseases can ruin the appearance of even a well-planned garden. New gardeners often don’t know about available disease control options, and just attribute the damage and loss of plants to having “a black thumb.” The result — they are less likely to buy more plants and continue gardening. Unfortunately, there are a host of diseases that can damage ornamentals, including leaf spots, mildews (powdery and downy), blights and a seemingly endless array of other afflictions.

One example is the fungal disease “petal blight” (Botrytis) that can be a serious problem on a wide range of crops. Although Botrytis is often classified by plant pathologists as a weak pathogen, under the right conditions it causes significant damage to both greenhouse and bedding plants. A period of cool moist weather can result in the type of symptoms seen in Figure 1. Symptoms typically appear first on the softer flower tissues; hence the name “petal blight.” However, infection can also spread to the leaves from the petals after they drop.

So what can you do about these diseases?

Of course, fungicides can be applied. Unfortunately, these have to be reapplied fairly frequently, depending on conditions. If you grow roses, for instance, you get used to explaining that the white stuff on the leaf isn’t a disease, but fungicide powder used to control disease — and no, that chemical aroma isn’t a new rose fragrance. Finally, although pesticides today are pretty safe, potential effects on pets and children are a worry you could do without.

How about breeding resistant varieties?

Different flower varieties can have different levels of resistance to Botrytis (for instance petunia, Figure 2). However, selective breeding for resistance is time consuming, and the outcome is often uncertain. For instance, we are not aware of any specific genes in petunia that have been used for breeding Botrytis resistance.

In spite of these potential drawbacks, traditional pest management and breeding remain important tools for variety improvement. However, application of biotechnology also offers promising opportunities. For example, an effort funded by the American Floral Endowment is underway in our group to introduce genes that increase resistance to fungal pathogens like Botrytis. Initial efforts use a gene for the enzyme mannitol dehydrogenase that was isolated from celery as part of an earlier study on the role of mannitol metabolism in stress tolerance.

We’re excited by these possibilities, not just because initial results are promising (see Figure 3), but by using a gene from a plant we eat (assuming you eat celery or parsley) to modify a plant you don’t eat, we avoid some of the common concerns associated with genetically modified plants. Our research has, in fact, found that this naturally occurring gene is expressed in many plants, but is apparently expressed too late or at too low a level to provide effective resistance in some cultivars. This is a fairly common occurrence. In our pursuit to enhance particular horticultural traits (color, size, etc.), we can lose others. Maybe that’s why we don’t stop and smell the roses any more; fragrance was sacrificed in pursuit of larger blooms and longer stems. Using modern biotechnology, perhaps now we have the opportunity to restore at least some of what we’ve lost.

About The Author:

John Williamson is associate professor and John Dole is professor of horticultural science at North Carolina State University. Williamson can be reached at [email protected] Dole can be reached at [email protected]

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