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Fertilizer rates and impact on root diseases

Ask any grower what the primary cause of root rot diseases is in their plants and they will tell you it is caused by overwatering.

While overwatering may enhance disease development it is not the sole factor. For any disease to attack a plant, three conditions must be present; first, the pathogen must be present. Second, the environment must allow for growth of the disease organism and finally, there must be a host plant that is susceptible to an attack by the pathogen. While over watering is a major contributing factor providing a favorable environment for root rot disease development, it cannot cause damage to the plant unless the plant is in a weakened condition and therefore susceptible to attack. Anything that contributes to "less than optimal growth" can result in a weakened plant, which is then more susceptible to disease organisms. Two commonly overlooked factors to poor growth performance are under and over fertilization.

What are the signs of under fertilization?
Under fertilized plants not only exhibit slow growth, but are weak in general and are at an increased risk of both diseases and insect attacks. Once attacked, plants will use additional nutrients in its attempt to fight off the disease. If plant nutrition is not quickly corrected, it can lead to compounding of damage. Under fertilization can be easily corrected by the application of water-soluble fertilizer which provides which provides nutrients that are immediately available for plants to use.

How to fix overfertilization? 
Overfertilization on the other hand can be much harder to correct, especially if the overfertilization is the result of high applications of Controlled Release Fertilizer (CRF). Over fertilization damages plants in several different ways. It often results in very “leggy”, soft growth of plants and high salt levels from fertilizers can burn delicate root tips. Both of these compromised conditions will result in weakened plants and increased susceptibility to disease attacks. If the cause of over fertilization is from elevated levels of water-soluble fertilizer, the situation can be corrected by leaching the growing medium with water to remove excess nutrients and then adjusting the rate of future fertilizer applications to meet the needs of the plants. Over fertilization when using CRFs can result from either the presence of too much fertilizer or the fertilizer releasing faster than expected because of high temperatures. In either case, there is no quick solution to removing the excess nutrients. The grower must leach with water on a regular basis to remove the excessive fertilizer as it is released from the fertilizer prill. In general, it is best to apply CRFs at a medium rate for the crop and supplement with water soluble fertilizer if additional nutrition is needed.

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