Mary M. Peet
Professor, Dept. of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609
mary_peet@ncsu.edu
A copy of this presentation, more information on diagnosing and preventing physiological disorders, and additional pictures of physiological disorders can be found on the NCSU greenhouse food production website: www.ces.ncsu.edu/greenhouse_veg/. Additional information on cultural practices, nutrition, and optimizing the environment is also available on this website.
Fruit disorders have both genetic and
environmental components, and in many cases, the exact cause of the disorder is
not well understood or a complex of factors is involved. Some fruit disorders
are also caused by insects or pathogens. The objectives of this article are: 1)
to present some general guidelines on optimizing the environment to prevent
disorders before they occur, with specific information on preventing ethylene
contamination; and 2) to help growers and crop consultants distinguish between
the various types of fruit disorders they are likely to encounter in the
greenhouse, sort out the probable causes, and offer possible corrections. Most
of this material refers specifically to tomatoes, but peppers also develop
blossom-end rot and cracking, and respond similarly to correction.
Pre-season:
Sanitation: Managing the greenhouse environment to
prevent fruit disorders starts before transplants come into the greenhouse.
Cleaning up plant debris from the previous crop reduces the incidence of
diseases like botrytis, which causes ‘ghost spots’ on green tomato
fruit. Cleanup also reduces the places where insects can hide. Feeding by the
silverleaf whitefly causes irregular ripening. Stinkbugs, thrips and mites can
also damage the fruit surface as they feed. Insects can also vector viral
diseases, such as the tomato spotted wilt virus, which cause mottling and poor
fruit color. Thorough sanitation of the greenhouse, including removal of
‘pet plants’ harboring insects, is the best way to get a good start
on the new crop.
Installation of insect screening over the vents and double
doors will reduce pest entry. Remember to follow the screening manufacturer’s
guidelines on making the screening box large enough so it doesn’t
restrict air circulation. Try to set up an antechamber or entry area so that
outside air is not sucked directly into the greenhouse every time a door is
opened. Pre-season is also a good time to train employees not to smoke in the
greenhouse, not to bring in contaminated material on clothes or plant material,
to scout for pest problems, and to report any suspicious plants promptly.
Checking out heaters: Another important pre-season activity is to check out the greenhouse heating and ventilation systems, especially if they have been modified, or if you have recently switched to wood heating. Heaters that produce ethylene or other combustion gases can cause flower abortion, completely preventing fruitset. The most common and serious forms of greenhouse pollution are combustion gases, such as carbon monoxide and ethylene, which are generated inside the greenhouse by faulty heat exchangers, dirty fuel openings and incomplete fuel combustion. Tighter, more energy efficient greenhouses, have added to the problem by reducing exchanges with outside air. At low concentrations, carbon monoxide (CO) can cause headaches and dizziness for workers. At more than 50 ppm (parts per million) (0.005%), injury and death can result for anyone in the greenhouse range. Plants are not particularly sensitive to carbon monoxide, so the main concern is with worker safety.
In contrast to carbon monoxide, ethylene is not dangerous to humans, except at high concentrations (10,000 ppm), but can devastate sensitive crops such as carnations, tomatoes and peppers. Ethylene levels less than 0.05 ppm can make tomato leaves twist downward (epinasty). This can be mistaken for water stress, except that the leaves feel turgid. With chronic exposure, stem thickening, increased branching, and plant stunting, as well as flower abortion can occur. These symptoms may be very subtle, and hard to diagnose at low exposure levels, especially if plants grown in clean air are not available for comparison. Problems can also persist long after the initial damage occurred, also contributing to the difficulty of diagnosis. If young plants are affected, and replacements are available, replacement may be advisable, rather than waiting for plants to ‘grow out’ of ethylene damage.
A characteristic of plant exposure to all types of air pollution, as opposed to plant disease, is that it may appear and go away suddenly, affecting both old and young foliage. Another difference is that ‘hot spots’ may correspond to air circulation patterns, rather than infection from a single plant. Many growers simply report that their crop ‘doesn’t look right’, or is growing more slowly than in previous years. Flower abortion is a good indication of ethylene problems. In the past, growers have been advised to buy cut carnations and bring them into the greenhouse to see if they wilt prematurely. However, now most cut carnations have been treated to retard senescence and may not respond to ethylene. Pots of miniature carnations, if available, might work as ethylene sensors. Bringing a few tomato transplants into the greenhouse before the entire crop is transplanted and observing them after running the heater overnight for a few days is a good precaution. Any twisting or wilting of the stems or leaves, especially if accompanied by flower abortion should be suspect.
For information on checking out heating and ventilation systems, and on ensuring that the right type of heating system is installed, consult the ethylene topic on www.ces.ncsu.edu/greenhouse_veg/. An article on ethylene damage to a number of crops (HIL 530) is also available on the floriculture website: http://www.ces.ncsu.edu/depts/hort/floriculture/crop/crop_ethylene.htm
Ethylene pollution can also result from sources as varied as compost piles in the greenhouse, welding equipment and liquid CO2 enrichment tanks. CO2 tanks approved for greenhouse use must not exceed certain limits of ethylene contamination, but cases have been reported where CO2 drawn from the top of the tank contained high ethylene levels. Ethylene is lighter than CO2 so the top of the cylinder or tank will contain a disproportionately high level of ethylene.
Cultivar selection: For almost all the of physiological disorders, cultivars differ in sensitivity. If a particular disorder, such as green shoulder, blotchy ripening or cracking, showed up last season, contact several seed suppliers and ask whether they have a cultivar which is less susceptible to this disorder. Of course, it is always advisable to trial a small amount of a new cultivar before converting over the whole greenhouse.
During the growing season: Diagnosing and correcting early
season problems
Twisted plants and leaves and aborted flowers, some yellowing of lower foliage: See information in the previous section on ethylene. If only fruitset is affected, check the overall plant health, and also ensure that adequate active bees are available for pollination. Also check for excessively high or low temperatures during flower development if plants appear healthy, but flowers are dropping off.
Misshapen or hollow fruit on the lower clusters (catfacing, rough, pointed fruit): Generally this is caused by cold temperatures down low in the canopy where the flower buds are developing. Pollen cannot develop and fertilization is reduced. Place a max/min thermometer or other sensors in the canopy at the height where flowers are opening. Make sure all temperature sensors are shielded from direct sun, which will cause them to read too high. Nighttime temperature should not go below10C (50F) to prevent fruit disorders, and should not go below13C (55F) for good fruit maturation and coloring and for overall crop growth. The recommended 24-h average for this period is 21C (70F). For fall crops, however, too high temperatures can also disrupt pollen production and release. Low light is also associated with hollow or puffy fruit and well as fruit with a mealy taste.
Ghost spot (small necrotic spots with a small whitish halo at the mature green stage of growth): Young developing fruit (1.5-3 cm or .6-1.2 inches diameter) develop these symptoms when a spore lands on the fruit, germinates, and then aborts. Other symptoms of botrytis are large, irregular brown lesions on the leaves and stems, which may have fuzzy, masses of gray-brown spores present. To prevent botrytis, remove pruned leaves and side shoots and other plant debris from the greenhouse promptly. Reduce condensation on plant leaves and fruit by adjusting the environment. Raising pre-dawn greenhouse temperatures slowly: 1C (2F) per hour, so that the daytime set point is reached an hour before daybreak will reduce condensation. Anti-condensate sprays on the interior greenhouse skin, or drip catchers which prevent condensation from the roof will also help. Improve air circulation by removing lower leaves, and running horizontal airflow fans. Procedures, such as venting and heating, which keep greenhouse relative humidity levels below 80% will also help. Keeping the area around the plant leaves warm will also reduce condensation.
Blossom-end rot (BER) is a common fruit disorder than can occur at any time in the growing season. Small, rapidly developing fruit are most likely to be affected and the disorder can appear when fruit loads are low, so I have included it as an early season disorder. It appears as a dry, sunken, black area at the blossom end of the fruit, and is sometimes confused with pathogen-caused diseases. Leaf scorch is a related disorder, in which young mature leaves exhibit marginal yellowing at the tips, and are sometimes distorted. It is promoted by some of the same factors that promote BER, but is more likely when fruit loads are high.
This disorder appears whenever calcium does not reach the tip of the developing fruit in sufficient quantities to support the building of cell walls. Calcium is transported to the fruit in the plant tissues carrying water. Lack of needed calcium can occur when plant transpiration is high. In this case, leaves are losing so much water that little reaches the fruit. In addition, calcium deficiency in the tissue can occur when levels of competing cations, such as ammonium-nitrogen, potassium and magnesium restrict uptake of calcium. Low root temperatures at night, salinity, and high daytime humidity may also restrict movement to the fruit.
Prevention includes ensuring that watering, media calcium and pH levels are in acceptable ranges. Leaf calcium from tissue analysis should be 2-4%, media calcium level should be 5-8 mmol/l, and plants should receive adequate irrigation. The media should not be allowed to dry. Plants are particularly sensitive to BER when a period of cloudy weather is followed by bright, sunny days. High relative humidity during the day also increases BER incidence. At night, however, factors reducing root pressure (high EC, low humidity) increase BER.
During the growing season: Diagnosing and correcting mid
season problems:
Nutritional, disease and pest-related problems can all appear during the peak of harvest. This is when pest problems have had a chance to build up and when the fruit load is putting heavy demands on the plant.
Fruit mottling or streaking: Plants producing discolored fruit should be tested for tomato spotted wilt virus (TSWV), which generally produces ringspots. Mosaic viruses, which result in necrotic blotches on the fruit and can also cause flower abortion if contracted during the fruitset period and can also be diagnosed by a lab. Generally there will be other virus symptoms on the leaves and stems, such as distortion and mottling. The presence of thrips in the greenhouse is a factor in spreading TSWV, so thrips populations should be monitored with yellow sticky traps and controlled if present, in addition to the preventative steps described earlier. Fruit discoloration can also result from feeding by silverleaf whiteflies. The grower will usually know whiteflies are present in the greenhouse, but it may be necessary to send a sample of adults and immatures to a diagnostic lab to confirm feeding by silverleaf, as opposed to greenhouse whiteflies. Both types of whiteflies will deposit honeydew on the fruit as they feed. Sooty mold grows on this honeydew, making the fruit difficult to sell. Only the silverleaf will cause fruit mottling, however. Control of the silverfeaf whitefly should reduce new fruit from being affected, but in the case of heavily infected greenhouses, it is not clear to what extent symptoms persist once whitefly feeding is stopped.
Fruit discoloration can also result from blotchy ripening, which is a poorly-understood physiological disorder. Coloring is uneven, with green or yellow areas on red-ripe fruit. Weather changes and any factors that affect growth rates can cause tissue dieback around the vascular bundles. Color development is poor around these areas. Blotchy ripening is sometimes linked to periods of reduced light with high temperatures. Boron and potassium deficiencies can sometimes be linked with this disorder. Suggested practices for prevention include increasing potassium feed by 20% from the two weeks before first picking until one week after first picking, maintaining EC at 3,0-4.0 during the summer, and reducing the amount of deleafing. Practices to balance plant growth, such as maintaining 20-23 fruit and 4-16 leaves per plant, may also help
Excess temperatures during ripening (above 30C or 86F) inhibit lycopene development. Lycopene is the pigment which gives tomatoes their characteristic red color. Fruits may color unevenly if part of the fruit is exposed to direct sunlight because skin temperatures on unshaded fruit can greatly exceed air temperatures. Direct temperature injury to the fruit is sometimes called sunscald or yellow neck. If the fruit on the top of the plant develops yellow patches, letting suckers or two head grow on the top of the plants may provide more cover. This disorder has also been associated with major temperature differences between day and night, low temperatures during the growth of the fruit, temperatures above 25(77F) when fruit is ripening or 22C (72F) after picking, and low potassium fertilizers.
Gold specks or flecks are often observed around the calyx and shoulders of mature fruit. In green fruit, the specks are white and less abundant. These specks decrease the attractiveness of the fruit and significantly shorten the shelf life. The specks are considered to be symptoms of excess calcium in the fruit. Dutch researchers found that under conditions of high air humidity and high Ca/K ratios, more calcium was transported into the fruit and the incidence of gold speck increased. Increasing the P level also increased calcium uptake rate and increased speckling. The disorder can be reduced by avoiding susceptible cultivars, raising the electrical conductivity of the nutrient solution reduced gold speck incidence, increasing the K/Ca ratio and increasing Mg. Presumably in all three cases, the mechanism was prevention of excess Ca uptake.
During the growing season: Diagnosing and correcting late season problems:
Fruit cracking: Cracks of varying size and depth occur in circles around the stem scar (concentric cracking) or radiate from the stem scar (radial cracking).
Cracking occurs when there is a rapid net influx of solutes and water into the fruit at the same time ripening or other factors reduce the strength and elasticity of the tomato skin. Increases in fruit temperature raise pressures of the pulp on the skin, resulting in immediate visible cracking in ripe fruit. In green fruit, minute cracks are created which later expand to become visible. High light conditions, especially on unshaded fruit have also been associated with fruit cracking. High light intensity raises fruit temperatures, fruit soluble solids and fruit growth rates, all of which factors are associated with increased cracking.
Cultural practices which result in uniform and relatively slow fruit growth such as constant, relatively low soil moisture, offer some protection against fruit cracking. In the greenhouse, reducing watering has been shown to decrease the incidence of radial cracking, and there are also a few reports in field crops of reduced cracking at lower levels of soil moisture. In field crops, cracking is usually attributed to fluctuations in the water supply. The classic occurrence is when a long period of drought is followed by heavy rain. Cultural practices that reduce diurnal fruit temperature changes also may reduce cracking. Greenhouse growers should maintain minimal day and night temperature differences and increase temperatures gradually from nighttime to daytime levels. Harvesting before the pink stage of ripeness and selection of crack-resistant cultivars probably offer the best protection against cracking.
Cuticle Cracking or russeting: In fruit with 'russeting', minute, hairline cracks, invisible to the naked eye, cover much of the fruit surface giving the fruit a rough feel. When examined closely the surface appears crazed and is said to have poor skin 'finish'. However, dull surface appearance or damage can sometimes also result from insect damage. At high levels, for example, spider mites can cause corky, cracked fruit. Clavibacter michiganensis can also cause small cankerous spots and radial splits on green fruit. Certain pesticides can also damage the skin, resulting in cuticle cracking as the fruit expands. Russeting increases under conditions conducive to other forms of cracking but is particularly associated with high humidity. It is not clear why cracking sometimes takes one form, and sometimes another.
In greenhouses, the following are thought to predispose plants to russeting: low fruit loads in strongly vegetative plants, low EC, a large previous harvest, large, slowly developing fruit, low light, and low temperatures during fruitset.
Poor fruitset: This can occur at any time. Early season, it is usually associated with low light and low temperature or ethylene pollution. Late in the season, however, it is usually associated with high temperatures. Recent studies have shown that mean daily temperatures over 25C (77F) when experienced by an individual flower for a week or more before opening, will reduce fruit size, seed content and fruitset. Under heat stress, buds may abscise prior to anthesis. This is particularly likely when there is a high fruit load already on the plant. Early indications are yellowing of the swollen ‘joint’ area, followed by yellowing from the joint to the flower. Although the flower drops off within a few days of this color change, the ‘stub’ remains. Flowers which experience heat stress sometimes fail to develop, but remain on the plant. These flowers should be removed, as the resulting fruit, if it grows at all, will be small and low in seed. If temperatures are excessive over a long period, parthenocarpic (seedless) fruit may develop.
Any
environmental or other condition that disrupts the normal course of pollen,
ovule or zygote development will predispose the flower or young fruit to
abscission. Typical causes of poor fruitset in the field or greenhouse are too
high or too low temperature or humidity; low light and high winds. For example,
day temperatures over 32C (90F) and night temperatures over 21C (70F) reduce
fruitset, as do temperatures below 10C (50F). As few as three hours at 40C (104F) on two successive days
may be sufficient to disrupt fruitset.
24-h average temperatures: 19C-21C (66-70) before fruit set; 17-18C (63-64F) for the first 6-7 trusses; 18-19C (64-66F) during heavy fruit production; 19-20C (66-68) late in the season (May-July).
Target VPD: 3-7 grams/m3, keep relative humidity below 85%.
For Further Information:
Dorais, M., Papadopoulos, A.P., and A. Gosselin. Greenhouse Tomato Fruit Quality. Horticultural Reviews.Vol 26. P. 239-319.
Greenhouse Vegetable Production Guide for Commercial Growers. Ed. J. Portree. Province of British Columbia Ministry of Agriculture, Fisheries and Food.
Kinet, J.M. and M.M. Peet. 1997. Tomato. In The Physiology of Vegetable Crops, ed. H.C. Wien. Commonwealth Agricultural Bureau (CAB) International, Wallingford, UK. 600 p.