With the persistent heat and dry conditions so far this summer, ginseng is stressed, and root development will be reduced. These conditions are also ideal for Alternaria development and spread. As a result, problems with Alternaria are continuing in many gardens.
A recent paper published by Dr. Mary Hausbeck’s lab at Michigan State University (Neils et al., 2020) on research conducted in Wisconsin has shown variable control with some of our most effective Alternaria products (e.g. fludioxonil (Scholar, Switch) and pyrimethanil (Scala)). There could be multiple reasons for the failure of these effective products to control Alternaria in some gardens including differences in disease pressure, weather conditions, and application timing. However, it is also possible that resistance is starting to develop in some of these products in some gardens.
While resistance has not been shown here, it is quite possible that it will occur if products are over used, and some individuals of Alternaria resistant to fungicides may already be present at very low populations. It is essential that Alternaria products be used as if these resistant individuals are already present, so they do not start to dominate the population.
Resistance of a pathogen to a fungicide develops through random mutations in certain individuals in the pathogen population. These mutations are very rare, and the chance of a mutation conferring resistance to a specific fungicide is also very rare even if a mutation occurs. For the sake of argument, let’s say that the chance of any mutation developing is one in a million, and the chances of that mutation causing resistance to a specific fungicide is also 1 in a million. That means the overall chance of a mutation developing that results in resistance to a specific fungicide is 1 in 1,000,000,000,000 individuals (a trillion or a million million). Seems like it is pretty much impossible for that occur, but look at this example:
- Each Alternaria lesion can produce around 500 spores per day (a guesstimate)(Figure 1), and every spore of Alternaria is a separate individual capable of multiplying
- Let’s say there is an average of 1 active lesion per plant in a 2-year-old or older garden over a 4-month period from May to September (some have many more, but many have no lesions at all)
- There are 100 plants per m2 in the average garden and 10,000 m2 in 1 hectare (=2.5 acres)
- There are approximately 3,000 ha of 2-year-old and older gardens (seedlings have minimal Alternaria)
So, 500 spores per day x 1 lesion per plant x 100 plants per m2 x 10,000 m2 per hectare x 3,000 hectares x 120 days per year = 180,000,000,000,000 spores per year (180 trillion). Although it seemed to be next to impossible for a specific mutation to occur, in this example 180 spores every year could be produced that have a mutation that results in resistance to a specific fungicide (and additional 180 resistant spores for every other fungicide). Many of these spores will not land on ginseng, since they blow randomly on the wind. However, some spores will land on ginseng and cause a new lesion, resulting in 500 spores being released every day that now have resistance to the fungicide.
Every time you use that fungicide, many of the susceptible lesions and spores will be killed, but the resistant ones will survive and continue to multiply. Eventually the resistant population is dominant, and the fungicide no longer works.
The chances of the resistant population also being resistant to another fungicide with a different mode of action is very low. If you switch to a new fungicide while the population of resistant Alternaria is still low, you can kill most of them and keep their population in check. However, if the new fungicide is not as effective it will not work as well to prevent resistance.
The old standby Alternaria products such as mancozeb (Dithane, Manzate, Penncozeb) and chlorothalonil (Echo, Bravo) are good for Alternaria, but as the research from Wisconsin shows, they are only effective when disease pressures are low. They are not as effective when disease pressure is high. That means that many lesions and spores of Alternaria will survive an application of these products under high disease pressures. If you have those products as the core of your program, and then only choose one highly effective product when disease pressures are higher (e.g. Scala, Switch, Flint), then the chances of resistance developing in your fields to that fungicide is very high. That is because the numerous individuals of Alternaria that survive mancozeb and chlorothalonil applications only face one fungicide. It is essential to have more than one and preferably many fungicides other than mancozeb and chlorothalonil with different modes of action within your rotation. Once resistance develops, it cannot be reversed, and that product will be gone forever.
Remember that Alternaria is a community disease, since spores are spread many kilometres on the wind. All it takes is one grower not following good fungicide rotation practices to result in resistance for the entire industry. It is in everyone’s best interest including chemical companies, suppliers, consultants and growers to ensure all growers are following good fungicide practices.
The Michigan State research has also shown the potential for the use of disease forecasting models to time fungicides for Alternaria more effectively, and reduce the number of fungicides required within a year. More research is required on this, but it shows that a calendar spray for Alternaria is not always required if conditions are not conducive to disease development at certain times of year.
Other than Alternaria and stresses due to heat, the ginseng crop is progressing as normal with no unusual insect pests or diseases being reported. Some areas received excessive rains in thunderstorms over the weekend, and these may be prone to spread of Phytophthora if wet conditions persist in repeated thunderstorms.
Reference: Neils, A.L., Brisco-McCann, E.I., Harlan, B.R., Hausbeck, M.K., Management strategies for Alternaria leaf blight on American ginseng, Crop Protection (2020), doi: https://doi.org/10.1016/j.cropro.2020.105302.