Fig. 1. Yellowing of foliage, or chlorosis, is one of the most common foliar disorders. This basil plant is exhibiting interveinal chlorosis from a micronutrient deficiency.
Photo: Christopher J. Currey

Pests and diseases can cause enough biotic problems for hydroponic producers. However, abiotic problems, those caused by non-living (abiotic) factors such as the greenhouse environment, plant culture, and imbalances in plant growth can also have a negative impact on our production by reducing productivity or quality. While there are a number of physiological disorders for hydroponic food crops, this article is going to focus on the most common physiological disorders of foliage, flowers and fruits you may encounter.

Foliage

One of the most common disorders found in lettuce is tip burn. The disorder is named after the necrotic tissue at the tip of leaves, most commonly observed on head lettuce. Tip burn is the result of a calcium deficiency in the rapidly-expanding leaf tips. A lack of calcium in the nutrient solution is one obvious cause of tip burn. If the only factor causing tip burn is a deficient nutrient solution, it can be remedied by adding calcium one or more sources to your nutrient solution. However, tip burn can also occur when there is adequate calcium in the nutrient solution, but calcium uptake does not occur due to reduced transpiration (water uptake). One of the most common factors that reduce transpiration and calcium uptake is high humidity. High humidity may be a challenge in the entire greenhouse environment, or in micro climates.

Tip burn is commonly observed in the center of heads, where the larger outer leaves reduce air movement around the center of the head, reducing air movement and increasing humidity around the head. Vertical airflow fans (VAF) can be used to reduce humidity in the centers of head lettuce, promoting transpiration and calcium uptake. If low light is diminishing transpiration, supplemental light can be used; increasing light intensity will enhance transpiration and calcium uptake.

Chlorosis is the yellowing of plant tissue (Fig. 1). There are three types of chlorosis: 1) across a whole leaf (general chlorosis); 2) between leaf veins (interveinal chlorosis); or 3) on leaf margins (marginal chlorosis). These are all common symptoms of nutrient deficiencies. The first step in avoiding nutrient deficiencies is to maintain the electrical conductivity (EC) and pH in your nutrient solution. If using an open (drain-to-waste) system, this entails correctly mixing fertilizer and pH adjuster stock solutions and regularly monitoring and calibrating fertilizer and pH adjuster injectors. For closed (recirculating) systems this entails the correct initial mixing of nutrient solutions and stock fertilizer and pH solutions, as well as maintaining correctly calibrated probes and peristaltic pumps which are used for automatic EC adjustment. In addition to maintaining the correct EC, you’ll need to perform nutrient solution analyses from a commercial laboratory to quantify the concentrations of individual nutrients, as imbalances in solution composition occurs over time as water and nutrients are taken up from the nutrient solution and clear water and fertilizer is added.

Why should we, in a section focused on foliage disorders, discuss flowering? The reason is that bolting and early flowering are problems when we are producing crops harvested for leaves, like lettuce, herbs and specialty greens. Bolting is a problem associated with head-forming greens like butter head lettuce. When head lettuces are beginning to flower, the normally compressed stem at the center of the head begins to elongate. As the stem elongates the heads become taller, while the foliage can develop a bitter flavor; between the changes in appearance and flavor, bolting makes lettuce unmarketable.

While some species or cultivars take exception to this rule, head lettuces generally grow best at cooler temperatures and high temperatures can cause these crops to bolt. Additionally, many greens flower in response to long days. As the days get longer and warmer, keep a close eye on your crop for bolting. Look for heat tolerant varieties, as they are less prone to bolting in production.

Fig. 2. This cucumber has too many young fruits on the plant to support developing them all into marketable fruits. Note one of the fruits on the left is senescing and will soon abort; the fruit load is too much for this plant.
Photo: Christopher J. Currey

Flowers

Flower abortion is problematic for fruiting crops, as reduced flower numbers can lead to lower yields. There are several causes of flower abortion. In the summer, high or supra-optimal air temperatures can cause flower abortion. When managing extreme summer temperatures, be careful not to use too much shade in an attempt to lower greenhouse temperatures; low light levels diminishes carbohydrate production.

Another cause of flower abortion is ethylene contamination. Although ethylene is a natural plant hormone, excessive concentrations from malfunctioning heaters damages flowers. In addition to flower abortion, ethylene damage may also cause leaves to curl down and inward (epinasty).

Tomato inflorescences (flower clusters) can revert from forming flowers to leaves. This reduces the number of potential fruits and, for tomatoes sold in longer clusters or scapes, reduces marketability. As tomato inflorescences develop new flowers, plants that are growing vigorously from excess water, nutrition, and/or light can exhibit reversion. This reversion of inflorescence development from reproductive to vegetative is the result of a strong source — a vigorously growing plant that is producing excessive carbohydrates and steering the plant from generative to vegetative growth.

We don’t want to decrease the light intensity to lower photosynthesis. Rather, leaf pruning and thinning out the canopy can reduce the overall source strength while optimal plant culture and environmental conditions can be maintained. Thinning canopies is most useful during the summer, when light intensities are greatest.

Fruits

Just like flower, fruits can be aborted. Like flower abortion, there can be several factors for fruit abortion. But the most likely cause is a source-sink relationship. Sometimes an excessive number of fruits can develop at the same time. When this occurs, there are not sufficient carbohydrates to support the development of all of the fruits; with insufficient energy, fruits start to abort (Fig. 2). To avoid this, aggressively manage fruit development and limit the number of fruits growing on a plant at any one time. This will not only reduce fruit abortion, but it will also improve the quality of the fruits that remain on the plant.

Uneven tomato and pepper fruit development is problematic because it makes fruits unmarketable. Misshapen fruits, or catfacing, is a disorder that both tomato and pepper fruits can exhibit. Fruits do not develop evenly and have a “puckered” appearance to them as a result of suboptimal pollination. When pollination is not complete and fewer seeds develop, the diminished levels of auxin are not sufficient for full fruit development. This can also happen when flowers are over-pollinated by bees, and the mechanical damage to the flower inhibits seed development.

Blossom end rot is a disorder that is observed on the bottom of tomato and pepper fruits. The disorder gets its name from the fact that the characteristic dark, watery tissue or necrotic spot develops on the end of the fruit where flower petals are stuck to as the fruit develops (the “blossom end”). This is another disorder that occurs in fruits that are deficient in calcium. Like tip burn, blossom end rot can be the result of inadequate calcium in the nutrient solution; as previously discussed, you can add calcium to your nutrient solution to fix this problem. However, like tip burn, blossom end rot can occur even when there is adequate calcium in nutrient solutions. Inadequate transpiration and calcium uptake can lead to blossom end rot. Unlike lettuce, VAF fans are not used for fruiting vine crops to increase transpiration. Rather, increasing light levels and/or reducing humidity can promote transpiration and calcium uptake. On the other hand, excessive transpiration can also cause blossom end rot. Very high light levels or low humidity levels resulting in excessive transpiration by leaves can diminish the amount of calcium allocated to the fruits, which have weaker transpiration and can’t compete with the demand of leaves.

Take-home message

Physiological disorders reduce profitability by diminishing yield and marketability of hydroponically grown food crops. The environment (light, temperature, and humidity), culture (water, mineral nutrition, pollination), and source-sink relationships in plants can cause foliage, flower, and fruit growth to go awry. Carefully managing your environment, culture, and production practices will help you avoid the disorders covered in this article and more.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu