Is worth a pound of cure. This old proverb, attributed to Ben Franklin, could have been taken directly from a modern plant pathology textbook. In today’s “business speak” we’d say, “It’s better to be proactive than reactive.” In either case, the lesson is clear: Anticipating a disease problem and taking action is far more effective than trying to manage the disease after it has become firmly established.
In the case of plant disease, prevention usually equals protection. The vast majority of our disease management tools — fungicides, in particular — work much more effectively when used as protectants. This means that the fungicide must be on the leaf or tuber surface before the plant is exposed to the pathogen. When used in the protectant role, fungicides prevent an infection from occurring in the first place by interfering with fungal spore germination and other early events in the infection process.
Protection is extremely important because, unfortunately, virtually all of our fungicides are ineffective on existing infections. After infection, the fungus becomes safely ensconced within the plant tissues where fungicide and fungus don’t come into contact. This is one reason why you can prevent but you cannot cure most fungal diseases.
Another reason to be proactive is that many diseases have an “incubation period,” which is to say that there is a time period wherein the plant has been infected but is not showing any symptoms as yet. Incubation periods are measured in days for some diseases and may last for weeks or even years in others.
This delay in symptom expression presents a two-fold problem: one, the infection that is causing the symptoms you are currently seeing may have occurred more than a week ago; two, there could be a lot more infection sites on your plants that are in their latent period today and look perfectly healthy but, unfortunately, they’re not, and symptoms will be showing up later. In other words, the disease you see could be a lot more serious than it looks like at first glance. Application of a protectant during the incubation period could give you the impression that your protectant has failed. This brings up another point: Application of a protectant during the incubation period will not cure the disease either.
In most production areas, for diseases like late blight and early blight, we have forecasting systems that inform growers when weather conditions have been favorable for these diseases to develop and that fungicide applications are recommended. The idea here is, once again, protection — to get the crop protected before the diseases have a chance to really get started. For diseases like white mold, a protectant application may be tied to some stage of plant growth, like flower drop, for instance. Still other diseases, like Rhizoctonia, require that the protectant be applied to the seed piece or in the furrow, long before the potatoes even begin to grow.
As you can see, for each of these diseases, the best management plan is to apply a protectant fungicide before the disease occurs. The use of a forecasting system, when such a system is available, can mean that a fungicide will not be applied until it is needed and that you might be able to get good disease management with fewer applications in some production seasons. But here’s the take home lesson: For plant diseases, an ounce of prevention is worth way more than a pound of cure because, most of the time, there ain’t no cure!