|
|
|
|
|
|
Physiological Disorders of Poinsettias These abnormalities are caused by non-biological factors, but could also be the results of efforts to control biological organisms, such as pests. In most instances, those effects of pesticides will be discussed in other sections of this publication. Nutrient deficiencies or toxicities are physiological disorders, but will not be discussed here with the exception of molybdenum, which has been suspected as being a cause of leaf deformities. High Temperatures The effects of temperature on growth and flowering are discussed in another section (see: Temperatures), but it should be mentioned here that improper temperatures cause several physiological disorders. Excessively high temperatures can delay flower initiation, and make scheduling difficult. Over 75% of the poinsettias produced in the United States are grown in areas where heat delay is a distinct possibility. Night temperatures in the mid- to high 70s (°F) can delay initiation, and such temperatures are not uncommon in late September and early October in the southeast. Some growers have tried to avoid the problem with variety selection but only a few varieties appear to respond favorably at such high night temperatures. The eventual solution of the disorder and undoubtedly will be the introduction of more heat-resistant varieties, but for now growers can only try to maintain night temperatures as cool as possible with evaporative cooling. It is known that the higher the night temperature, the shorter the daylength must be for initiation, so 9 or 10 hours days might ensure initiation, rather than reliance on natural daylengths in late September and early October. Control measures should be observed which prevents the maintenance of high temperatures under the shading material. |
|
|
|
Cool temperatures can also delay initiation and development of flower parts. The impact of cool temperatures can readily be seen in a greenhouse where temperature control is not uniform and there are cold and hot areas within the greenhouse. Adequate heating facilities and horizontal air fans can alleviate this problem.
Sales and delivery of plants during periods of extremely cold temperatures can be difficult because poinsettia bracts can be severely damaged by temperatures below 50°F. Red bracts develop a blue to a silver-white color and the extent of discoloration is dependent on length of exposure to such chilling temperatures. In some areas of the country, retail outlets are not accustomed to protecting poinsettias from chilling injury. Sleeving will help prevent chilling injury.
|
|
Physiological Disorders of Foliage This disorder most often occurs in the early stages of the crop. Plants develop distorted or cupped leaves. Most poinsettias will outgrow this condition, but shoots with extreme distortion may not improve. It is unclear what causes this disorder. Some distorted leaves are very symptomatic of molybdenum deficiency in other crops, and some research studies have shown that a lack of this element early in the life of the plant can result in such leaf malformations. There have been reports that deformities occurred on some plants even when molybdenum had never been deficient, so other factors could also be responsible. Mechanical injury to very young leaves can cause leaf distortion. Wind, water stress, and insects (thrips) are also a few ways such injury could occur. Rapid changes in humidity, as what occurs early in the morning when the vent fans come on, can lead to an accumulation of salts along the leaf margins and veins - resulting in leaf injury. Leaf distortion becomes apparent as these injured leaves grow and expand.
|
|
The most common cause of wilting is a lack of water, and application of water should correct the problem. If plants remain wilted, the root systems should be inspected to determine if root injury has occurred because of excess soluble salts, root rot pathogens, or fungus gnat larvae. Growing medium testing laboratories and plant disease and insect clinics should be utilized for proper identification of the reasons for the wilting.
|
|
Flowering plants which have been sleeved and shipped to the retail outlet might show symptoms of wilting when sleeves are removed and the plants have been in the retail outlet for a couple of hours. The first response is for the florist or store personnel to apply water, but moisture might not be lacking. This wilting is referred to as epinasty. Varieties differ greatly in their susceptibility to wilting. Mechanical injury or bending of leaves during sleeving can increase the amount of ethylene being produced by the plant. Increased levels of ethylene can lead to droopy plants. Length of time in the sleeves also has an impact on the extent of the epinasty. Growers should avoid rough handling of the plants during sleeving and the amount of time that the plants stay in sleeves should be minimized. There are several possible reasons for the development of shoots which are so weak and thin that the inflorescences cannot be kept erect without staking. Research has found that cutting quality plays a part in stem breakage. Larger sized cuttings do not tend to break as easily as smaller, weaker cuttings. Early in the production cycle the plants should also be spaced closer together so that the plants will grow more upright. Plants grown with a wider spacing are more likely to produce lateral shoots that can grow out horizontal over the edge of the pot. Support rings will help support the plant and prevent stem breakage, but the cost of the rings and the labor to install them has to be economically justified by the grower. Calcium deficiency also has been reported by some researchers as a cause of weak stems, as calcium is an important constituent of plant cell walls. Varieties can also differ in stem strength with 'Success' and 'Red Splendor' being varieties which are more resistant to breakage. Some of the free-branching varieties produce so many shoots that crowding and reduced light intensity occur. The number of shots can be controlled to a certain extent by limiting the number of nodes below the pinch to 5 or 6 or by the removal of some of the lateral shoots. The first method is practice more often, since pruning can be an expensive operation. Limiting the percentage of ammonical-nitrogen being applied may also help avoid stem breakage. Ammonical-nitrogen promotes vegetative growth that may lead to weaker stem. Since thicker, stronger stems generally result for growth regulator treatments, some growers apply growth regulators to improve stem strength as much as to control height. |
|
Crowding of plants late in production can reduce light intensity among the inner rows of plants on the bench and lateral shoots will be weak and spindly. Also plants provided with ample spacing, but grown with a root substrate in which the plant was growing remained so wet that irrigation was only required every 3 or 4 days can lead to weak stems.
|
|
A major attribute of modern varieties is that they are not as prone to leaf abscission as were earlier ones. Lack of water can cause leaf drop, but as wilting first occurs, any alert grower should notice the water deficit before the shortage becomes acute enough to cause leaf drop. It is more likely to occur in the home office, mall, or wherever the poinsettia is placed by the customer. Irregular irrigation, low-light intensity, warm temperature, and low relative humidity make leaf drop a common disorder.
|
|
Most chlorosis problems are caused by nutrient deficiencies (see: Nutrition section). Cycocel spray applications also can cause chlorosis (yellow spots), however, particularly if the growth retardant is applied at high concentrations (1:40 or 1:50 [above 1,500 ppm]) and when the greenhouse environment is hot and bright. Excessive application rates can cause interveinal chlorosis and leaf cupping. To prevent damage, a lower concentration of Cycocel should be applied with repeat applications being used. The injury usually occurs early enough in the development of the crop that injury is not evident when the plants are sold. Severity of the problem can be lessened if weaker concentrations are used, and applied in early morning or late afternoon. Early applications, perhaps, are best, as late applications result in wet foliage during the night, increasing the chances for Botrytis, particularly in double-layered plastic greenhouses.
|
|
The splitting disorder was first encountered with the variety 'Paul Mikkelsen', and it has been a problem with some other varieties since them. It can occur on stock plants or the finishing plants. It is especially frequent on stock plants if the shoots become long, with numerous leaves, before a pinch or removal of a cutting is made. It can occur even when the daylength is too long for floral initiation, but a floral primordium is produced and is then surrounded by lateral vegetative shoots. Splitting in stock plants is almost like an uncontrolled pinch, and growers should be wary of cuttings taken from such shoots. Cuttings taken early in the propagation season and grown too long as single-stem plants are more likely to split than those taken later.
Splitting can occur on plants which are to be sold as flowering plants, and can make the plants unacceptable. Again, this is more likely to occur on plants which are grown single-stem with a mature main axis, compared to plants of the same variety which are grown as pinched plants, with the younger lateral shoots. Several ways have been recommended to reduce the changes of splitting. These methods are:
|
|
Bract necrosis/Bract Edge Burn This disorder, which can be recognized by brown bract margins and eventual internal necrosis, and also referred to as bract burn occurs most frequently on the varieties 'Gutbier V-14 Glory' and 'Supjibi'. Generous fertilizer applications, particularly ammonium sources of nitrogen, continued into the late stages of the season, seem to increase the likelihood of bract burn. Research also has shown increased incidence of the problem when calcium was deficient. Excess soluble salts in the growing substrate causing root injury, reduced water absorption or stress from inadequate or excessive irrigation, damage from pesticides or pollutants, and high relative humidity are also associated with the disorder. Perhaps the greatest damage caused by bract necrosis in the increased chance for Botrytis infection of the damaged tissue, and then the need to control this persistent disease.
The following suggestions have been made:
|
|
This disorder, once referred to as crud does not seem to be as prevalent as it was several years ago on varieties popular then, but occasional cases are reported. Latex, which erupts in the shoot apex when cyathia are forming, dries and seems to physically prevent the continued development of the flower parts. Malformed inflorescences reduce the quality and acceptability of the plants. Latex also can erupt from leaf surfaces, prompting a grower to consider mealybug damage, but damage to the shoot apex is much more serious. High relative humidity and generous amounts of water in the substrate seem to increase the chances for latex eruption, so avoidance of the disorder is geared to the control of these factors.
|
|
The bracts are the conspicuous features of the poinsettia and the true flower parts in the center of the apex are relatively unnoticed unless they are missing. Varieties differ in their ability to retain cyathia, and 'Gutbier V-10 Amy' was an example of one where the cyathia abscise rather quickly. Often, the cyathia fall off when the plants are still in the greenhouse. However, many of the newer cultivars have improved cyathia retention or there are few cyathia present, which limits the use of cyathia as an indicator of age. A combination of high night temperatures (70°F or higher) and low-light intensities during the day, have been shown to increase the chances of premature cyathia abscission. Research has shown that such conditions result in an inadequate supply of carbohydrates, and cyathia do not have a high priority for the carbohydrates which are available. Inadequate water also increases the severity of the problem. Maximum night temperatures of 65°F until early November, followed by temperatures of approximately 60°F, and proper spacing of the plants to allow light penetration into the interior of the bench are recommended procedures to reduce the incidence of cyathia abscission. Water stress should not be allowed to occur. Varieties, particularly those red bracts, differ in the intensity of the pigmentation, but sometimes the color of the bracts on finished plants does not meet expectations. Bract color might be referred to as faded, though failure of the pigments to develop properly is perhaps more accurate. High night temperatures can be responsible for the lack of color intensity, and explains why night temperatures of approximately 60 to 62°F are recommended from early or mid-November until marketing. Bracts which are produced under crowded conditions, with low light levels, will usually be lighter in color than bracts in the upper canopy of the same plant. This disorder occurs to plants on the edge of the bench. Brushing against the plants can cause the bract tissue to turn white where excessive contact has occurred.
Ethylene (C2H4) is an odorless, colorless gas which acts as a plant hormone, a growth regulator, and a potentially harmful pollutant of ornamental crops. Poinsettias demonstrate an interesting wilt like appearance (epinasty) after exposure to ethylene gas. Leaf epinasty has been observed when poinsettias were exposed to 10 ppm ethylene. Epinasty can be observed on poinsettia plants when they are kept in their shipping sleeves for a prolonged time. Petioles of poinsettias naturally produce ethylene in response to sleeving. Deformed top growth can also occur during long term exposure to ethylene. Preventing economic losses due to ethylene can be achieved by avoiding exposure to engine exhaust from shipping trucks and other combustion engine vehicles, ripening fruit, senescing plant materials, smoke, welding fumes, and poorly maintained greenhouse furnaces. Annually servicing boilers and burners may reduce or prevent ethylene damage to floricultural crops. Gas leaks resulting from cracked heat exchangers may allow harmful concentrations of ethylene to be released into the greenhouse. Continual expansion and contraction of the metal in the heat exchanger of a furnace can stress the welds resulting in cracks. Leaks at joints and seams can be discovered by painting soapy water on them. Another method of detecting leaks is the placement of smoke bombs or furnace candles within the firebox. Light or smoke penetrating from the interior should cause alarm to growers.
|
||
|
Home : Plant Part / Problem Type : Plant Structure / Problem Location : Symptoms / Cause |
||
|
|
