By: Jackie Carroll
A wet growing season is bad news for an onion crop. Read on to find out about onion diseases and their control.
It’s difficult to tell the difference between the many diseases affecting onion plants. Even the experts often have to rely on laboratory tests for a definitive diagnosis. Fortunately, you don’t have to know exactly which disease has infected your plants to take action.
Onion plant diseases arise during warm, moist weather and most have similar symptoms, which include spots and lesions on leaves and bulbs, areas that look as though they are water-soaked, browning foliage and toppling. There is no method of treating diseases of onion, and you can’t reverse the damage. The best course of action is to focus on next year’s crop so that it doesn’t happen again.
Here are some growing tips to help prevent the introduction of diseases into your onion crop:
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Perfectly healthy plants can be affected by air, water or soil borne diseases at any time. Understanding the causes, identifying them early and taking effective action will help you establish and maintain a healthy vegetable garden.
Anthracnose. This is a fungal disease present in Europe since the 1990s affecting tomatoes, beans and cucumbers and many other plants. It causes lesions on plant stems, leaves and fruit which develop into spore groups during warm, wet weather. Affected plants should be destroyed.
Anthracnose disease attacks all plant parts at any growth stage. The symptoms are most visible on leaves and ripe fruits. At first, anthracnose generally appears on leaves as small and irregular yellow, brown, dark-brown, or black spots. The spots can expand and merge to cover the whole affected area. The color of the infected part darkens as it ages. Infected fruit has small, watersoaked, sunken, circular spots that may increase in size up to 1.2 cm in diameter.
Organic treatment is a copper type spray similar to that used to control potato blight below.
Blight. Potatoes and tomatoes are affected by this fungal organism. Leaves will initially shrivel and rot before the disease spreads to the fruit which will turn brown and decay. Blight thrives in warm, wet weather commonly in late summer.
Early varieties of potato will normally crop before these conditions arrive. It is good practice to rotate seed stock or to use blight resistant varieties. Affected plants should be destroyed and not composted.
Club Root. Affecting brassicas, the first signs of infection are wilting, blueish leaves and a dying plant. Roots become swollen and distorted, restricting growth and yield. Alkaline soils can help to curtail, although not eliminate, the disease.
The disease is will be brought into a new garden with infected plants, it is common in allotment gardens where traffic between plots is high but isolated gardens rarely have it.
Once you have the disease in the garden you will have to live with it as the spores remain in the soil for up to 9 years. You can avoid growing brassicas in the garden for some time or take the following precautions: Lime the soil the previous Autumn (club root thrives in acidic conditions), use raised beds (clubroot likes wet conditions), burn or otherwise dispose of brassisca roots rather than adding them to the compost.
Damping Off Disease. Damping off is caused by a variety of different soil borne fungi causing young seedlings to rapidly fail. Plants under stress due to high temperature or waterlogging are particularly prone.
Using good quality seed compost, thorough cleaning of pots and trays and good ventilation will help to prevent this condition. Slight underwatering at seedling stage is also more advisable as plants create a stronger root system searching for water and are far less likely to be troubled by damping off disease.
Downy Mildew. This appears as a white downy growth on the undersides of leaves and along stems. It can be avoided by improving air circulation to eliminate the damp, humid conditions that cause it. Selective pruning can help with this to improve air flow.
Water in the early morning, avoid splashing the leaves and give plants time to dry out during the day. Severely infected plants should be removed. If plants are grown in a greenhouse a soil irrigation system can be beneficial as it reduces the need for foliar watering and keeps the leaves dry.
Fusarium Wilt. This fungal disease spreads from the plant root to the capillary vessels in the stem, impairing the plant’s ability to draw up water. The plant will start to wilt and leaves will turn yellow, progressing from the older foliage to the new as the growing season develops.
When plant stems are cut lengthways the brown striped of diseased capillaries will be readily apparent. Suspected plants should be removed and disposed of as early as possible and in severe cases compost should be replaced.
Leaf Spot. This disease is present in a bacterial or a fungal form and can affect most plant species. It appears as a black or brown spot, often with a yellow halo, which will spread and eventually destroy the affected leaf. The disease is common in warm, moist conditions and will spread easily when watering. There is no remedy other than preventive care. Fungicides can be used when early symptoms appear, particularly with brassicas.
Mosaic Virus. The leaves of beans, tomatoes, and peppers, are affected by mosaic virus, displaying mottled green and yellow foliage. Leaf curl and wrinkle occurs and plant growth is often stunted. Preventive measures are encouraged, such as planting resistant varieties and deterring pests, especially aphids and leafhoppers, which spread the disease. Remove and destroy any infected plants.
Powdery Mildew. This fungus specific to each host plant displays a white powdery coating on the surface of leaves and fruits which causes plants to become distorted and growth to die back. Affected plants also often display dark brown or bright yellow spots.
If it isn’t treated, the problem can cause plants to die or fruiting to fail. Removal and disposal of infected leaves and stems will help prevent the development of spores for the next season.
Rusts. Common rusts can affect everything from asparagus to beans, carrots, and onions. Infected plants develop reddish brown powdery spots on leaves and stems. Rust diseases are not fatal but may result in damage to the foliage preventing flowering and fruiting. Prevent infection by providing good air circulation around crops and remove any seriously affected plants.
Most diseases, particularly fungal based, can be prevented by exercising good gardening practices. Choosing certified seed and rootstock will ensure your plants have a good start and carry a good degree of resistance to many diseases. Be diligent in cleaning tools, trays, pots and anything that comes in contact with your plants and soil. Clean and disinfect your greenhouse at the start of the growing season to eliminate any problems remaining from the previous year. Sensible watering and good ventilation will help prevent the growth and spread of many fungal organisms.
Many bacterial, fungal and viral diseases attack vegetable crops in Maryland home gardens. Most of these are not serious and in very few cases is spraying a fungicide recommended. Remember, when disease symptoms are noticed it is usually to late to spray a fungicide. Below are twelve tips that can help you prevent disease problems:
When disease symptoms are observed it is often too late to apply a fungicide, although fungicide treatments can help to protect new or un-infected foliage. Fixed copper, sulfur, and horticultural oil are some organic fungicides used by home gardeners. Always, carefully read and follow all pesticide label information and test the spray on a small part of the crop to check for signs of leaf injury (phytotoxicity.)
Onions grow best on fertile, well-drained soils. Tifton series 1 and 2 soils are found in the Vidalia onion area and are well suited for onion production. However, most sandy loam, loamy sand or sandy soils will be advantageous to sweet onion production. These soils are inherently low in sulfur, which allows greater flexibility in sulfur management to produce sweet onions. Avoid soils with heavy clay content and coarse sandy soils. Clay soils tend to have a higher sulfur content, which can lead to pungent onions. Exceedingly sandy soils are more difficult to manage because they require more fertilizer and water.
Fertilizer and lime requirements should always be based on a recent, properly obtained soil sample. Check with your local county extension office or crop consultant regarding proper procedures for soil sampling and interpretation of results. Obtain the soil sample a few months prior to crop establishment in order to determine lime requirements and make necessary lime applications in a timely manner. If soil test results show a pH below 6.0, apply and disk in dolomitic lime two to three months before land preparation to bring the pH to the optimum range of 6.2 to 6.5. It is essential to apply sufficient lime to keep the soil pH above 6.0. Low pH can cause nutrient deficiencies during the growing season. Also, the high rates of fertilizer used in producing onions cause the pH to drop during the growing season. If the pH is not corrected at the beginning of the onion season, nutrient deficiencies could occur during the year and reduce yield. Calcium and phosphorous deficiencies can often be linked to low pH, even though soil tests indicate adequate levels. But phosphorus deficiencies due to low pH can be difficult to correct during the growing season.
Onions require more fertilizer than are used in most vegetable crops because fertilization of both plantbeds and dry bulb onions must be considered. They respond well to additional fertilizer applied 40 to 60 days after seeding or transplanting. The method of fertilizer application is very important in obtaining maximum yield with multiple applications insuring good yields. This will increase the amount of fertilizer utilized by the plant and lessen the amount lost from leaching. More recent research however, indicates that good results can be obtained with as few as three fertilizer applications. Preplant fertilizer will vary with the natural fertility and cropping history. Proper application methods and function of various nutrients are outlined below. Table 3 shows a suggested fertilizer program for a soil testing medium in P and K.
Nitrogen (N), especially in nitrate (NO3) form, is extremely leachable. If too little nitrogen is available, onions can be severely stunted. High nitrogen rates are believed to produce succulent plants that are more susceptible to chilling or freezing injury, disease, and to production of flower stalks. Onions, heavily fertilized with nitrogen, are believed to not store well. Finally, excess nitrogen late in the growing season is believed to delay maturity and causes double centers. Make the final nitrogen application at least four weeks prior to harvest. Rates of nitrogen will vary depending on soil type, rainfall, irrigation, plant populations and method and timing of applications. For dry bulb production from transplanting or direct seeded onions should require between 125-150 lbs/acre nitrogen. It is usually best to incorporate 25% to 30% of the recommended nitrogen prior to planting and apply the remainder in two or three split applications.
Phosphorus (P) is essential for rapid root development. It is found in adequate levels in most soils but is not readily available at low soil temperatures. Because of these factors, under most conditions all of the P should be applied preplant and incorporated before transplanting. This amount should be counted as part of the total seasonal fertilizer application. Table 4 shows the recommended phosphorous to be applied based on various soil test levels.
Potassium (K), is an important factor in plant water relations, cell-wall formation and energy reactions in the plant. Potassium is also subject to leaching from heavy rainfall or irrigation. Therefore, it is best to split K applications by incorporating 30% to 50% of the recommended K before planting and splitting the remainder in one to two sidedress applications. A low K level makes plants more susceptible to cold injury. Table 4 lists recommended K applications based on soil test results.
Sulfur (S) is an essential element for plant growth. Early applications of sulfur are advisable in both direct-seeded and transplanted onions. To minimize pungency, fertilizers that contain S should not be applied after the end of January. Research conducted in Georgia on S and onion pungency has shown that pungency (pyruvate analysis) of mature onions increases with high rates of S or whenever S applications are made after late January. Therefore, S should not be applied to onions after late January unless the onions exhibit S deficiency. Do not completely eliminate S from the fertility program. Apply 40 to 60 pounds of elemental S with half incorporated at transplanting or seeding and half applied at the first sidedress application. Do not apply S in rates higher than 40-60 lb/acre.
Boron (B) is required by direct-seeded or transplanted onions in the field. If the soil test shows B levels are low, apply one pound of B per acre and incorporate prior to transplanting or seeding. Do not exceed the recommended amount since boron can be toxic to onions.
Zinc (Zn) levels determined to be low by soil testing can be corrected by applying five pounds of Zn per acre. Excessive amounts of Zn can be toxic, so apply only if needed. Zinc is usually added in the preplant fertilizer.
Magnesium (Mg) levels in the soil must be adequate for good onion growth. If dolomitic limestone is used in the liming program it will usually supply some of the required Mg. However, if soil pH is adequate and the soil-test Mg level is low, apply 25 pounds of Mg per acre in the preplant fertilizer.
Slow release fertilizers have been introduced to the Vidalia growing region. These fertilizers have performed well and can be considered in a fertility program. These fertilizers however, have not proven satisfactory for single fertilizer application.
Table 3. Sample fertilizer recommendations for transplanted onions with a plant population of 60,000 to 80,000 plants per acre. Make adjustments for soil test levels other than medium P and medium K.
The red eggs are from the European Red Mite. Because I have never encountered them personally I can't really offer advice as to how to control them. However I do know that they are eggs which will hatch out in spring and become tiny red mites.
These will damage the leaves of your apple tree. I would certainly scrape the eggs off immediately before they hatch and take a close look around the tree for more of them. When the eggs hatch they loose their colour so this will give you an idea of the scale of the problem this year.
To the best of my knowledge the European Red Mite is not connected with the more common Red Spider Mite. More information can be found on the link here.
Insect Pests : Insect pests can be divided in to 3 main groups :
1. Chewing insects
2. Sucking insects
3. Boring, mining & soil insects
Grasshoppers : Camouflage easily on plants due to their green or brownish colours. These insects cut irregular cuts and punctures on leaves. Tender shoots too are eaten. Their droppings are like tiny pieces of charcoal. As they hop from plant to plant, grasshoppers may be or may not be present on the damaged plant.
Brown tussock caterpillar : It is a larva of a moth which lays its white eggs in clusters, usually on the underside of leaves. The tiny, hairy caterpillars in a flock start scraping leaves. This makes the affected leaves look translucent. As they grow, they start nibbling the leaves, which get cut irregularly. Tender shoots, flower buds too are affected. Droppings of the caterpillars are globular.
Chafer beetle : This beetle too causes irregular cuts and puctures on leaves. This insect is active in rainy season. The chafer beetle is a nocturnal creature and causes the damage only at nighttime. During the day they hide in the soil.
Lemon Butterfly Caterpillar : This common and beautiful butterfly lays its solitary eggs on leaves of citrus plants. The tiny caterpillars, which hatch out of the eggs, resemble droppings of sparrows in colour and shape. Leaves and new growth is all chewed up. As the caterpillar grows bigger, it acquires green colour and then easily camouflages amongst the foliage.
Leaf Tier Caterpillar : This pale green caterpillar with black head is a larva of a moth. Plants of ginger family are targeted by this caterpillar. As the name implies, this caterpillar rolls and ties a leaf edge with silky secretion. Under the rolled edge, the caterpillar hides and eats leaves.
Semi Looper : These wire thin caterpillars are experts in camouflaging. They are always of the same colour of the plant on which they feed. Apart from the colour of their body, when they detect some trouble, they stand erect on their hind lags and as a result look very much like a leaf stalk. Unlike other caterpillars, a semi looper walks by forming a loop between its extremes and then stretching.
Aphids : These winged or wingless, tiny insects flock in great numbers on tender shoots and flower buds and suck the sap. They flock and breed in such profusion that they cover the entire stems and flower buds. They are more common in winters. Aphids attack most cultivated plants. Apart from causing the damage by sucking the sap, they are also capable of infecting the plants with viral, bacterial and fungal diseases.
Scales : Brownish irregular scaly growth on branches of hibiscus plants are actually colonies of brown scale insects. These are soft bodied insects with brown or blackish colour, having spine-like outgrowth on their bodies. Hard scales, resembling mini shields, cover entire stems of rose plants. All scale insects are capable of movement in their juvenile stage only. The hard scales insects, as they settle on plants to suck the sap, start secreting a waxy substance. This Wax covers their bodies entirely, thus making them immobile and impregnable from contact type of insecticides.
Mealy Bugs : This common wooly and white insect attacks most plants in a garden. Due to their fungus-like appearance and immobility, many people confuse them with fungus. However, below the wooly growth, they have extremely soft, pink bodies. Like aphids and scales, they too flock in great number on plants. These insects attack all parts of a plant, even the roots.
Thrips : The thrips are so tiny that they can not be seen easily. They are winged and are capable of flight. Thrips cause immense damage to tender shoots and flower buds. Thrips have saw-like mouth parts with which they rasp the parts of plant and suck the juice. The affected parts look as if they are burnt. These insect are more common in hot summer days. As the damage caused by them resembles sunburns, people unaware of the presence of these insects, normally confuse the damage to be of sunburn.
Jassids : They are also known as leaf hoppers. These tiny, winged insects are common in monsoon. Very often, attracted by lights, they come in houses at night. They are about the size of a wheat grain and the colours vary from green to brown.
Red Mites : Mites are not insects. Mites are tiny spiders, red or brown in colour. They attack most plants. Their presence is indicated by dusty formations on the affected leaves. This is due to the dust, which settles on the webs spun by these tiny creatures. The affected leaves also show numerous colourless spots. This is due to constant sucking of plant juices.
Leaf-Cutting Bee : This is a type of a bee and is comparatively harmless. This bee cuts neat, circular or oval pieces of leaves only along the leaf edge. The pieces so cut are rolled to form a nest to lay the eggs. The leaf-cutting bee does not chew plant parts, neither does it suck sap.
BORING, MINING & SOIL INSECTS
Citrus Leaf Miner : This minute caterpillar mines irregular shaped and glistening galleries in citrus leaves. Due to its habit of hiding inside the leaf, it is impervious to contact insecticides.
Ixora Caterpillar : These tiny brown caterpillars attack new shoots of Ixora and also the buds and flowers. The caterpillar bores through buds and as it eats, it secretes a waterproof web around the affected area. The webs are full of the caterpillar’s droppings.
Digger Wasp : Tunnels on pruned stems of a rose plant are made by these tiny wasps. The tunneled stem dries and dies. This tunnel could harbour fungal or bacterial infection like die back or canker disease.
Gall causing Caterpillars : The swollen outgrowths on a stem of Bhendi or Tondli creeper are caused by a tiny caterpillar. The affected stems get deformed and then wither and die.
Bark Eating Caterpillar: Unlike other caterpillars, this attacks only the matured, woody trees. The caterpillar starts eating the bark first but gradually bores through the wood and travels upwards in the branch. The bored branch ultimately dies. Continued infestation can kill a giant tree.
Ants : Ants are capable of damaging the seeds. Sometimes they carry off the seeds sown. Due to their tunneling in ground, they are capable of disturbing roots of saplings, but otherwise they do not harm plants directly. Some sucking insects like aphids and mealy bugs secrete sugary drops. These sugary drops attract the ants. Ants help the sucking insects by carrying their (sucking insects’) young ones to new sites. As the branch dries due to constant sapping of juices, the sucking insects, which are almost immobile, perish along with the branch. This would deprive the ants of their favourite food. Thus the ant help the insects in return for the sugary drops.
White Grubs : These are larvae of beetles. Both larvae and the beetle are pests. Grubs gnaw at roots and the beetles chew the greens. Adult beetles lay their eggs in decaying matter. Their infection starts normally through farmyard manure or compost pits.
Snails and slugs : These pests affect only the damp areas in a garden. Mostly they shelter in the soil in the day time and feed on plants by night. They leave a sticky glistening trail as they crawl. Snails have shells on their backs and slugs have no shells. They are bisexual creatures and can reproduce even without mating. In dry spells they go in to hibernation and wake up when seasons are favourable.
Millipedes : Millipedes normally eat decomposed matter in soil. However, if starved of food, they may devour roots.
Pill Bugs : Pill bugs are soft bodied insects, which look like tiny cockroaches. They feed on tender hair roots. When disturbed, they roll in to a ball shape hence the name pill bugs.
All insects are not pests. One should know the insects, which are beneficial to mankind. Dragonflies, damselflies, praying mantis, lacewings and lady beetles feed on other insect pests. Bees help in pollinating flowers, upon which fruiting and seed formation depends. Using insecticides indiscriminately can cause harm to the beneficial insects too
Insecticides can be divided in 3 types depending upon their mode of action on the insects. Contact insecticides are capable of killing an insect, which merely comes in contact with a plant on which a contact insecticide has been sprayed. However, many insects, which bore through the parts of plants or cover their bodies with protective webs or waxes can not be killed by contact insecticides. To kill such insects, systemic insecticides are very useful. Systemic insecticides, when sprayed on plants, circulate through entire system of plant. Thus, any insect, in spite of its clever shelter, is killed when it sucks the sap of the plant or chews any part of the plant. Fumigant insecticides give off fumes, which are capable of killing insects.
Contact insecticides give a superficial layer of protective coating on surface of plants. In heavy rains, this coating is washed off hence in monsoon they are not very effective. Certain part of a plant, which has not been treated could harbour some insects without harm. But contact insecticides have certain advantages too. They are quicker to act. They are capable of killing an insect, which merely comes in contact with a plant treated.
Systemic insecticides once absorbed by the plant circulate through all parts of plant. Thus, even the insects eating the roots would get killed, when only upper parts of the plant are sprayed. As the plant absorbs these insecticides, they do not wash off even in heavy rains. Since leaf pores are located on underside, while spraying of systemic insecticides, care has to be taken to see that the spraying is done mainly on the underside of the leaves. In case there is no spray pump, systemic insecticides could be applied near the roots also. One drawback of systemic insecticides is that they will not kill the insect, unless the insect chews a part of a plant or sucks the sap of the plant.
Plant diseases are either non-infectious or infectious. Non-infectious diseases are caused by very low temperatures, mineral excesses and deficiencies. Infectious diseases are caused by parasitic organisms like fungi, bacteria or virus pathogen. Diseases caused by fungi are easier to control than the bacterial and viral diseases.
Fungal Diseases: Rust, black spots , powdery mildew, seedling damping off, Coconut crown rot are some of the fungus diseases.
Rust : Plants affected with rust, get rust coloured patches on leaves and other parts.
Black spots: In winter and in monsoon, dewdrops or raindrops remaining on foliage encourage this fungus disease. Small brownish-black spots appear on the leaves, which get enlarged with yellow margins. The affected leaves turn complete yellow with black spots and the fall down.
Powdery Mildew : This disease too infests plants in monsoon or in winter. Affected parts get white powdery patches. The patches gradually turn yellowish. The affected leaves fall down. Spores of this fungus are airborne and thus spread easily. In downy mildew the fungus growth is hairy.
Seedling Damping Off : Over-watered seedlings rot near soil surface and collapse. This disease can be prevented but the seedlings affected can not be saved.
Coconut Crown Rot : This fungus disease is caused mainly because of rhinoceros beetle, which chews and bores through a crown of a coconut palm. The fungal infection spreads through the wounds created by this beetle. Once the crown rots, it is impossible to save a coconut palm. By controlling rhinoceros beetles this disease can be prevented to quite some extent.
Bacterial Diseases : Canker disease is caused by bacterial infection. The affected parts form corky outgrowth. Stems tend to crack open. Rose plants and citrus plants are quite susceptible to this disease. In lemon plants leaves, stems and even the fruits get the smallpox-like corky outgrowth.
Viral Diseases : Most viral diseases are incurable. Seasonal plants and short life plants like Banana and Papaya affected with viral diseases need to be destroyed. Aphids are vectors of many viral diseases. By controlling aphids, viral diseases can be prevented to some extent. Bunchy top of Banana, leaf curl of Tomato and brinjal, Chilli and Papaya, yellow mosaic leaf of Bhendi are some of the virus diseases.
Weeds : Any plant growing in an unwanted place is a weed. Amongst the weeds, grasses are more difficult to kill. Leaves produce food for plants. Constant removal of leaves can starve a plant to death. So by regular removal of above-ground parts can kill a weed. Covering weeds with black polythene sheet also can kill them. The plants thus covered do not get sunlight for photosynthesis. The extreme heat generated under the black polythene sheet can also kill many weeds. The polythene sheet also prevents new weeds from sprouting. Weedicides are of three types. General weedicides will kill all weeds which come in contact with it. The broadleaf weedicides will kill all plants except grasses. Grass weedicides will kill only grasses but not the broadleaf plants. All weeds must be exterminated before they form seeds else seeds dispersl will make eradication impossible.