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Fertilizer and Disease: What is the Connection?

Ginseng growers have seen many foliar diseases in the past few weeks. These diseases have been due to favourable weather conditions and frost damage. However, the role of fertilization in the development of disease is often overlooked. No fertilization strategy can prevent disease when weather conditions are highly favourable for infection, and fertilization cannot prevent frost damage. However, over the life of a garden, improper fertilization can make disease outbreaks more severe than they otherwise would be. While very little research has been conducted to test the effects of fertilization on ginseng, research has shown some consistent effects of certain nutrients on similar diseases in other crops.

There are 3 main ways that fertilization can affect disease:

1. By altering the microclimate within the canopy due to changes in plant size and shape
2. By directly affecting the infection, growth and reproduction of the pathogen
3. By increasing or decreasing the plant's natural defences
   

Low Fertilization

Too little of a nutrient can weaken the plant and make it more susceptible to certain diseases. This is especially the case for diseases that are only weak pathogens and can only penetrate a weakened plant. For example, Alternaria diseases are often increased by low fertilization, especially nitrogen. However, much of the nitrogen requirements for ginseng comes from the breakdown of manure and organic matter. Even if applied nitrogen rates are low, the plant still may have sufficient nitrogen to produce a healthy crop. The recommended nitrogen rate for ginseng is 40 kg/ha broadcast each spring. Low calcium, which is often related to acidic soil, can also increase disease.

Excess Fertilization

Excess fertilization can often cause a boost in plant growth, which results in larger and more succulent plant cells with weaker cell walls. This can cause some pathogens, especially root rotting pathogens such as Phytophthora and Pythium, to more easily infect the plant. Research has shown that damage from many root rotting fungi increases with increasing nitrogen application. However, inconsistent effects of fertilization have been found for Rhizoctonia and Fusarium diseases. While laboratory studies have shown that Cylindrocarpon develops faster with higher nitrogen, this has not been confirmed in the field.

The other problem with too much fertilization is the excessive leaf growth that often occurs. This can cause a very dense canopy and restrict air movement. As a result, the leaves can remain wet for a longer period of time after rain or dew periods, and humidity can be higher. This can increase infection periods for foliar diseases and also restrict the penetration of fungicides into the canopy. Foliar Phytophthora and Botrytis can increase under these conditions.

So how do you know how much to apply? Recommended rates of many different nutrients are provided in Publication 610: Production Recommendations for Ginseng. Soil and tissue analysis can also provide some clues to whether there is a deficiency or excess of nutrients. When fertilizing ginseng, keep the following key points in mind:

• Proper fertilization can reduce disease susceptibility of ginseng.
  
• It is possible to have too much of a good thing - avoid over-fertilizing.
  
• No fertilizer can completely prevent disease development - combine proper fertilization with cultural and chemical disease controls.
  
• Ensure proper fertilization before planting because it can be very difficult to correct deficiencies of some nutrients after they occur.