Gordon Johnson, Retired Extension Specialist; gcjohn@udel.edu, Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu
As we move into cole crop production season, pay attention to factors that can cause physiological disorders in broccoli, cabbage, and cauliflower. Managing nutrients and irrigation to minimize plant stress and using stress tolerant varieties are key to avoiding many of these physiological disorders.
Hollow Stem in Broccoli and Cauliflower Not Caused by Boron Deficiency
This condition starts with gaps that develop in the tissues. These gradually enlarge to create a hollow stem. Ordinarily, there is no discoloration of the surface of these openings at harvest but both discoloration and tissue breakdown may develop soon after harvest. Some cultivars of hybrid cauliflower and broccoli may have openings from the stem into the head. Both plant spacing and the rate of nitrogen affect the incidence of hollow stem. Hollow stem increases with wider spacing and as the rate of nitrogen increases. The incidence of hollow stem can be greatly reduced by increasing the plant population.
Cabbage Splitting
Cabbage splitting is mainly a problem with early cabbage. A problem can develop when moisture stress is followed by heavy rain. The rapid growth rate associated with rain, high temperatures and high fertility causes the splitting. Proper irrigation may help prevent splitting and there are significant differences between cultivars in their susceptibility to this problem. Splitting may also be partially avoided by deep cultivation to break some of the plant roots.
Cauliflower and Broccoli Buttoning
Buttoning is the premature formation of a head and because the head forms early in the plant’s life, the leaves are not large enough to nourish the curd to a marketable size. Buttoning may occur shortly after planting in the field, when normal plants of the same age should be growing vegetatively. Losses are usually most severe when transplants have gone past the juvenile stage before setting in the field. Stress factors such as low soil nitrogen, low soil moisture, disease, insects, or micronutrient deficiencies can also cause this problem. Some cultivars, particularly early ones, are more susceptible to buttoning than others.
Lack of Heads in Broccoli and Cauliflower
During periods of extremely warm weather (days over 86°F and nights 77°F) broccoli and cauliflower can remain vegetative (does not head) since they do not receive enough cold for head formation. This can cause a problem in scheduling the marketing of even volumes of crop.
Yellowing and Purpling in Cauliflower
The market demands cauliflower which is pure white or pale cream in color. Heads exposed to sunlight develop a yellow and/or red to purple pigment. Certain varieties such as Snow Crown are more susceptible to purple off-colors, especially in hot weather. Self-blanching varieties, which have leaves that cover the head tightly, have been developed to reduce problems with curd yellowing and purpling. Other newer varieties do not have tightly wrapped heads but are less prone to developing yellow or purple head coloration. Newer orange cauliflower and green broccoflower varieties are being planted. They are less susceptible to off-color but still can develop purpling under warm conditions.
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Purpling developed on this head of cauliflower where wrapper leaves left some of the curd exposed to light.
Alcala and Twister are varieties with contrasting levels of head wrapping. Twister has leaves that wrap tightly around the head, but it is prone to purpling if the head becomes exposed to light.
Cauliflower Ricing
“Riciness” and “fuzziness” in heads is caused by high temperatures, exposure to direct sun, too rapid growth after the head is formed, high humidity, or high nitrogen. “Ricing” is where the flower buds develop, elongate and separate, making the curd unmarketable.
Cauliflower head that has developed “fuzziness”.
Development of Curd Bracts in Cauliflower
Curd bracts or small green leaves between the segments of the curd in cauliflower is caused by high temperature or drought. High temperatures cause a reversion to vegetative growth with production of bracts on the head. In a marketable cauliflower head, the individual flower buds are undeveloped and undifferentiated.
Cauliflower heads with different levels of curd separation and curd bract formation in response to heat stress.
Loose Heads in Cauliflower and Premature Flowering in Broccoli
Loosely formed curds in cauliflower can be due to any stress that slows growth making them small or open. Fluctuating temperatures and moisture will also cause less compact growth. In contrast, excess vegetative growth caused by excessive nitrogen can also cause loose heads in cauliflower and broccoli. Premature flowering and open heads in broccoli can be brought on by high temperatures.
Edema on Cole Crop Leaves
Edema is water blistering on Cole crop leaves. The most common cause of edema is the presence of abundant, warm soil water and a cool, moist atmosphere. Under these conditions the roots absorb water at a rate faster than is lost through transpiration. Excess water accumulates in the leaf, some parenchyma cells enlarge and block the stomatal openings through which water vapor is normally released from the plant; thereby contributing to further water retention in the leaf. If this condition persists, the enlarged cells divide, differentiate a cork cambium, and develop elongate cork cells externally to form a periderm. The rupture of the epidermis by the enlarged inner cells and the periderm account for the raised, crusty appearance of older edema spots.
Black Petiole
Black petiole or black midrib is an internal disorder of cabbage that has been occasionally noted in recent years. As heads approach maturity, the back side of the internal leaf petioles or midribs turn dark gray or black at or near the point where the midrib attaches to the core. The affected area may be quite limited or may extend for 2 or 3 inches along the midrib. It is believed that this disorder is associated with a potassium (K)-phosphorus (P) imbalance and results when the K level in the soil is low and the P concentration high. High rates of nitrogen may contribute to the problem. Probably, as in the case with tip burn, black petiole is a complex physiological disorder in which environmental conditions play an important role in symptom expression. Variety evaluation trials have shown that there are differences in degree of susceptibility between varieties.
Floret (Bead) Yellowing or Blonding in Broccoli
The florets are the most perishable part of the broccoli head; yellowing may be due to over maturity at harvest, high storage temperatures after harvest, and/or exposure to ethylene. Any development of yellow beads ends commercial marketability. Bead yellowing due to senescence should not be confused with the yellow to light-green color of areas of florets not exposed to light during growth, sometimes called “blonding”.
Brown Floret (Bead) in Broccoli
This is a disorder in which areas of florets do not develop correctly, die and lead to brown discolored areas. This is thought to be caused by excess heat during head initiation but also may be due to feeding damage on florets from insects such as harlequin bugs.
Brown bead in broccoli