by Hunter Landis (Agronomist at the North Carolina Department of Agriculture – Plant, Waste, Solution and Media section) and Brian Whipker (professor of floriculture at North Carolina State University)
Interveinal chlorosis is readily identified as a problem associated with inadequate levels of iron (Fe) (Fig. 1). Many factors affect Fe uptake. In greenhouse production, elevated substrate pH, which makes Fe unavailable to the plant, is usually the primary suspect (Fig. 2).
Figure 1. Interveinal chlorosis of the upper leaves most times indicate that iron is limited. (photo: Brian Whipker)
Also keep in mind a number of other factors can also negatively impact Fe uptake by plants. Hydrangeas (Hydrangea macrophylla) are often grown with cooler temperatures. As a standard rule, substrate temperatures below 55F (13C) limits Fe uptake by the plant.
Figure 2. The interaction of substrate pH on nutrient availability. At higher pH levels greater than 6.5, iron is tied up. (photo: Brian Whipker)
In addition, at these lower temperatures, water use by the plant can be less, thus making the plants more susceptible to overwatering. Iron (and phosphorus) uptake is a challenge for hydrangeas, like most plants, with excessively wet conditions. Prolonged substrate wetness inhibits root development, which can exacerbate the situation already occurring in the plants.
The impact of these cultural factors of cool temperatures and saturated substrates can be managed by careful water management. In practice it may be difficult due to plant-to-plant variation. One will need to irrigate when the majority of the crop requires water. If possible, the plants that dry out slower may need to be grouped and irrigated at a less frequent interval.
From a plant diagnostic perspective, symptoms are often visible on a larger percentage of the crop when Fe is limited due to low temperatures or overall wet substrate conditions (Fig. 3). If a Fe deficiency is suspected, then submit a substrate and tissue sample to a commercial lab for confirmation.
Figure 3. Interveinal chlorosis of the upper leaves of only a few plants suggest either a cultural problem or root rot is occurring. Inspect the roots for poor growth or root rot to help diagnose the situation. (photo: Brian Whipker)
Root rot is another factor that needs to be considered if upper leaf interveinal chlorosis is occurring. Root rot problems usually occur on scattered, individual plants and the new growth is usually stunted. So the scattered pattern of stunted plants will aid in your diagnosis of a root rot problem. In addition, when leaf interveinal chlorosis occurs, always inspect the roots. Black or brown roots are a strong indicator of rot. Submit a sample to a disease diagnostic clinic to confirm your diagnosis.
In this situation we analyzed the interveinal chlorotic symptomatic leaves and a sample from dark green leaves as a “good” comparison. The tissue analysis results confirmed that Fe was limited (Fig. 4). The good comparison plants had 121 ppm Fe, while the leaves with initial to moderate interveinal chlorisis symptoms contained 78.4 ppm Fe. Plants with advanced symptoms of bright yellow leaves contained 37.7 ppm Fe in the tissue. The recommended range for Fe varies by source. Bryson and Mills (2014) list a range between 50 and 300 ppm Fe, while Dole and Wilkins (2005) have a narrower 85 to 115 ppm Fe range. The plants exhibiting initial to moderate upper leaf interveinal chlorosis were within the acceptable range suggested by Bryson and Mills, but below the recommended range of Dole and Wilkins. The leaves with more advanced symptoms were below the recommended ranges of both references.
Figure 4. A comparison of tissue iron (Fe) values from a lab analysis. Low Fe levels were confirmed by the test. (photo: Brian Whipker)
In this situation of observing interveinal chlorotic leaves scattered throughout the crop, root rot was suspected. The roots were then inspected and were white and healthy, but limited root growth was noticed. The interveinal chlorosis was likely confounded due to the previous week of cloudy weather, which limited the frequency when fertilizer was applied, and resulted in prolonged substrate wetness.
We took one of the plants back to our greenhouse to grow it longer and observe the symptoms. By customizing the irrigation frequency based on the need of this individual plant, the plant was able to recover on its own without applying any acidification corrective procedures. The leaves turned green within 2 weeks.
This example illustrates the interaction between slow root growth, environmental conditions, and excessive irrigations. With plant growth, it is impossible to decouple them, thus in order to avoid this situation, all these factors must be addressed when taking corrective steps.
Literature cited
- Bryson, G.M. and H.A. Mills (editors). 2014. Plant Analysis Handbook IV, Micro-Macro Publishing, Athens Georgia. p. 600.
- Dole, J.M. and H.F. Wilkins. 2005. Floriculture Principles and Species, 2nd Ed. Pearson-Prentice Hall, Upper Saddle River, NJ. p. 1023.