How to tell a nutrient deficiency from a disease Is it
Fusarium on your tomato plants or is it a nutrient deficiency? Sometimes it is hard to tell. In tomatoes, growers can mistake the yellowing of older leaves as a Fusarium problem when it could be caused by a nitrogen deficiency. Nutrient deficiencies, explains Stephen Reiners, an associate professor in the Department of Horticulture at Cornell University, are often related to soil moisture, the alkaline and acid levels in the soil, temperature of the soil, and other issues.
To determine exactly what is going on in the field, however, he says growers should have their soil tested at least every three years. “For anywhere from $10 to $20, a grower will know if his nutrient levels are adequate, what needs to be added, and if they know nothing else, the soil pH,” he explains. “The pH tells us whether the soil is too acid or too alkaline.
For tomatoes we like to see a slightly acid pH, between 6 and 6.5. Lower values may decrease the availability of phosphorus, calcium, and magnesium while higher values lead to phosphorus, manganese, boron, and zinc deficiencies.” Reiners recalls a visit he made a few years ago to a grower who was producing a few acres of staked tomatoes on plastic mulch using trickle irrigation. He was asked to visit the farm because the fruit was poor quality and showing internal yellowing. “The grower had spent all that money to grow his crop yet had not invested a few dollars in a soil test,” he says. “We tested the soil that day and sure enough, potassium levels were very low.” A Big Difference In most situations, says Reiners, nutrient deficiencies will differ from diseases in many ways. Typically, a nutrient deficiency is more uniform across a field and will affect different species equally. As a disease moves into a field, it first may appear more sporadic, impacting a portion of the field before moving on to the rest of the field. “It will likely impact one commodity and not another as it is species specific,” he says. Nutrient deficiencies, he continues, do not cause soft rots. Instead, they induce browned, dried leaves. “Diseases often result in soft rots and water soaked spots,” Reiners explains. “Diseases also may cause wilting of plants as the water-conducting elements in the stems become clogged. Deficiencies don’t normally cause wilting. Often in the center of a yellow or brown spot will be another brown spot (Septoria) or a target spot (Early Blight).”