Starved for attention

Fertilizer effects on diseases have been researched for well over 100 years. Before fungicides were really effective and available, it was one of the only ways to influence disease severity. Even today, fusarium wilt is more readily controlled with fertility and pH management than with fungicides. While controlling disease in container ornamentals is easier to do than in a field situation, fertilizer trials have been performed almost exclusively in soil and not in soilless media. The trials performed were rarely on ornamentals. Finally, trials most often looked at a single factor and not all interactions between nitrogen, phosphorus and potassium (main targets). To a lesser degree chlorine, copper, aluminum, calcium and silicon have also been researched. Once an effect is consistently identified, researchers determine what causes the reduction or increase in disease severity. Many times these effects are indirect by changing the plant response and not a direct effect on the pathogens.
 

Know your nitrogen

Both the level and source of nitrogen has been found to be very effective in reducing fusarium wilt. Generally, fusarium root rot and wilt is increased with use of ammoniacal nitrogen. It is also increased with low pH (which occurs with ammoniacal nitrogen). In contrast another wilt disease caused by a different fungus, Verticillium, is increased with high pH and increased with use of nitrate nitrogen. This is the opposite of the reaction of fusarium wilt making an accurate diagnosis very critical. Black root rot, caused by Thielaviopsis, is also increased with high pH and use of nitrate nitrogen. Interestingly, the results are consistent for the specific disease and plant. Unfortunately, results differ by pathogen on the same plant or by plant for the same pathogen.
 

Other major nutrients

Results are inconsistent with phosphorus. In contrast, potassium improves host resistance perhaps through thicker outer cell wall development. It also may reduce disease by allowing the plant to outgrow the pathogen. Botrytis leaf spot, vascular wilt and downy mildew are generally decreased as potassium increases. Pythium root rot increases while rhizoctonia root rot decreases with increased soil potassium.
 

Minor nutrients

Chlorine generally suppresses diseases (especially rust and fusarium wilt). Rates that suppress disease have minor toxic effects on the pathogen and the effect appears to be indirect by changing the host plant. Copper decreases many diseases caused by bacteria and fungi. The effect is generally not due to direct toxicity to the microbes, although this plays a role on superficial pathogen populations. Copper (and many other minerals) influence the plants defense systems and therefore aide in disease control.

Aluminum is toxic to plants and microbes, and most studies were performed in soil and rarely soilless media. Black root rot on tobacco decreased as exchangeable aluminum increased. Disease occurred less often with a pH of 5 or lower.

Increased calcium decreases post-harvest diseases of roses and fruit like apple, carrot, grapefruit, melon, nectarine, peach and potato (before and after storage). Calcium also reduces root rot cause by Phytophthora and Pythium. Calcium in recirculated water systems can reduce disease caused by Pythium or Phytophthora if they reproduce via zoospores.

Silicon applied to plants has been touted as disease control. While it can work on plants called silicon accumulators, not all plants accumulate silicon. Silicon accumulators have greater than 1 percent silicon by dry weight. If the plant is not an accumulator then increasing silicon in the fertilizer/potting medium will not have any effect on silicon in the plant or disease response. Plants that are silicon accumulators include monocots like rice, grasses (turf and wheat) and cucurbits including cucumbers and melons.
 

Fusarium wilt control with mineral nutrition

Research over the past 100 years consistently shows that using certain forms of nitrogen and maintaining soil/medium pH are still the most effective means of reducing fusarium wilt.

Fusarium oxysporum varieties infect the vascular system of specific crops (often only a single genus). For instance the pathogen causing fusarium wilt on tomato does not infect any other crop. Symptoms include yellowing, wilting and collapse due to clogging of the xylem and production of toxic compounds that damage the plant. Mineral nutrition is the only thing that works on fusarium wilt (cotton, tomatoes, banana, gladiolus).

The methods for control of fusarium wilt in the first half of the 20th century showed consistent and good control when crops were fertilized with lower levels of nitrogen and when the nitrogen was derived from nitrate and not ammonium sources. Work has been performed on mums, carnations and hebe in containers as well as tomatoes, cotton and bananas in the field. Thus higher pH can reduce spore production and therefore disease spread and development.

Additional experiments have shown that increasing pH to 6.5-7.5 can reduce severity of fusarium wilt as well.

Some studies suggest that when pH is higher, many minor elements are less available. These are required by Fusarium oxysporum to make spores.

First and foremost, supply nutrients in a balanced way to the plant. Second, know the main disease issue of your crop since not all diseases respond the same way to mineral nutrition. Supply nutrients at the right rate and right time for the crop to use. For fusarium wilt especially, choose the nitrogen form to minimize disease. Finally, modify the environment to make nutrients available to the crop. In many situations, stressing the crop can result in higher levels of disease.

A.R. Chase is the part-owner of Chase Agricultural Consulting LLC and Professor Emeritus of Plant Pathology at the University of Florida. Learn more at www.chaseagriculturalconsulting.com.