GROWER 101: Nutrient Disorders in Greenhouse Crops
Four sets of information are important when assessing thenutritional status of greenhouse crops and developing corrective procedures.First, determine irrigation water quality prior to developing the location. Dothis during a wet and a dry period shortly after establishing the business andperiodically thereafter, preferably annually. Based on these results, changesin the fertilization program can be made to compensate for water-qualityproblems such as high alkalinity, high EC, imbalance of calcium (see April 2002issue of GPN) to magnesium or high levels of individual nutrients such asboron. Second, root substrate tests should be conducted during each crop tomonitor substrate pH and EC and the availability of nutrients. Third, a foliaranalysis should be conducted to determine the levels of nutrients successfullytaken up.
If these three tests are run and properly interpreted onschedule, it should be possible to make the required periodic shifts in afertilization program to avoid nutritional disorders. Unfortunately, this doesnot always occur. When a nutrient disorder occurs, a fourth set of information,visual symptomology, becomes very helpful. The following are common symptomsassociated with deficiencies and the more common toxicities of nutrients ingreenhouse crops. Look for more in the next issue of GPN.
Nitrogen Deficiency.The older leaves become uniformly chlorotic. After considerable time, olderleaves become necrotic and drop off if abscission is possible for the speciesin question. Purple to red discoloration may develop in older leaves as theyturn chlorotic in some species such as begonia, marigold and pansy.
Phosphorus Deficiency.The plant becomes severely stunted, and at the same time, the foliage becomesdeeper green than normal. In some species, the older leaves develop purplecoloration. Older leaves then develop chlorosis followed by necrosis. Rootsbecome longer than normal when the deficiency is moderate.
With foliage plants, older leaves may lose their sheen,becoming dull green followed by red, yellow and blue pigments showing throughthe green, particularly on the undersides of the leaves along the veins. Thesesymptoms spread across the leaf. Older leaves abscise if possible. Otherwise,necrosis develops from the tip toward the base.
Magnesium Deficiency.Older leaves develop interveinal chlorosis. In several species, pink, red orpurple pigmentation will develop in the older leaves following the onset ofchlorosis.
In foliage plants with pinnately (netted) veined leaves,bronze-yellow chlorosis begins at the upper margins of older leaves, progressingdownward along the veins, leaving a green, v-shape pattern at the top of theleaf. As chlorosis progresses down the leaf, a green, v-shape of tissue remainsat the bottom. Eventually, the tip, and then the base, become chlorotic.Necrosis follows chlorosis in the same pattern.
Iron Deficiency.Young leaves of seedlings sometimes develop general rather than interveinal chlorosis.In late stages, the leaf blade may lose nearly all pigment, taking on a whiteappearance.
Iron Toxicity. Thisdisorder mainly affects African marigolds, seed geraniums, basil, cosmos,dahlia, nasturtium, pepper, strawflower, tomato and zinnia. Marigolds developbronzing on recently fully expanded leaves. The bronzing consists of numerouspinpoint spots that begin yellow and quickly turn bronze. Affected leavesbecome necrotic. Older leaves on the other crops develop numerous pinpointnecrotic spots across the blade. As the spots enlarge, they turn necrotic untilthe entire leaf dies.
Boron Deficiency. Symptomsinclude incomplete formation of flower parts such as fewer petals, smallpetals, sudden wilting or collapse of petals and notches of tissue missing inflower stems, leaf petioles or stems. Death of the bud giving rise to branchingis followed by death of the new buds, eventually leading to a proliferation ofshoots termed a “witch’s broom.” Short internodes, crinklingof young leaves, corking of young leaves, stems and buds, and thickening ofyoung leaves all occur. Chlorosis affects young leaves but not in any definitepattern, resulting in eventual death of the root tips of short and thick roots.
Additional symptoms in foliage plants can include : brittlestems and leaves; necrotic spots (black and sunken) on stems just below nodes;nodal roots on vine plants that may become thick, short and abscise; and vinesthat may become highly curled at the nodes.
Boron Toxicity. Themargins of older leaves become necrotic with a characteristic, reddish-browncolor. Necrotic spots may also develop across the leaf blade but tend to be concentratedat the margins.
Molybdenum Deficiency. Symp-tomsapply to poinsettia, the only greenhouse floral crop it is known to affect. Themargins of leaves at the middle of the plant become chlorotic, presenting asilhouette appearance and then quickly becoming necrotic. Symptoms spread upand down the plant. These leaves may also become misshapen, resembling ahalf-moon pattern with some crinkling.
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