As follow up to our April 1 update, problems with iron inefficient crop species continue. In the last week, symptoms of iron deficiency were reported for both snapdragon and Calibrachoa crops. A characteristic symptom of iron deficiency is interveinal chlorosis on young and developing leaves. Since iron is required for chlorophyll synthesis, the yellowing is caused by the emergence of young leaves that arise despite the absence of enough iron to support healthy green color.
Both snapdragon and Calibrachoa species are included in a group typically referred to as the ‘iron inefficient’ or ‘iron sensitive’ bedding plant species, because of reduced capacity to obtain iron from growing media relative to other crops. As such, these species are especially prone to iron deficiency.
Since iron is less available to plants at higher pH values, and more available at lower pH values, the principle strategy for preventing iron deficiency in iron inefficient species is to maintain media pH in the range of approximately 5.5-6.2. This is accomplished through practices such as attention to starting pH of growing media, and careful and consistent monitoring of pH levels throughout the crop cycle. Tools for influencing pH levels include management of alkalinity in irrigation water, and the use of fertilizer sources which, depending on formulation, may help to either lower or raise substrate pH.
Other tools for preventing and managing iron deficiency problems in iron inefficient species include optimizing phosphorus fertility (excess P can reduce iron availability due to the formation of insoluble iron phosphates) and preventive and/or curative applications of chelated iron.
While iron deficiency in iron inefficient crops is a common occurrence, when symptoms arise it is important to consider other possible issues that may be in play before taking action:
- Recent weather conditions have been predominantly cool, overcast, and wet. Under such conditions, transpiration is slower, which translates to slower uptake of nutrients (including iron) from growing media. Less transpiration also means that media remains wetter for longer periods of time, which reduces root activity and thus nutrient uptake. These factors together can cause or exacerbate iron and other nutrient deficiency problems. Cool, wet, transpiration-limiting conditions also can delay plant response from applied fertilizers as well as the impact of applied fertilizers on pH values.
- Reduced irrigation during extended stretches of cool, overcast weather can cause EC levels to rise, and elevated EC can sometimes result in chlorosis. Check EC frequently and take corrective action if needed.
- Media that remains wet for longer periods increases the potential for root infecting fungi, such as Thielaviopsis, to take hold. Roots compromised by disease are much less effective at obtaining nutrients from growing media, which can result in deficiency symptoms. Be sure to check roots often.
- Conditions that limit transpiration can also limit boron uptake, leading to symptoms that can approximate iron deficiency, such as chlorosis on young leaves.
- Bedding plants affected by tobacco mosaic virus (TMV) can display yellowing and mottling that can appear very much like iron deficiency symptoms.
Regular monitoring of greenhouse crops is essential for staying ahead of emerging problems and minimizing potential losses. For guidance in adjusting growing parameters like pH, EC, alkalinity, and nutrition, UMass Extension’s Soil and Plant Tissue Testing Lab provides both media and plant tissue testing. UMass Extension’s Plant Diagnostic Lab offers analysis for pests and pathogens.
For more information:
How to Prevent Iron Deficiency in Iron Inefficient Greenhouse Crops: https://ag.umass.edu/greenhouse-floriculture/fact-sheets/how-to-prevent-iron-deficiency-in-iron-inefficient-greenhouse
Report by Jason D. Lanier, UMass Extension Specialist
Photo by Jim Mussoni, Private IPM Scout