Beetle Damage

 

 

 

 

Botrytis

 

Botrytis is widely distributed around the world on flowers, fruits and vegetables.

Transmition

Transmitted when the light intensity is low and the humidity high.

Symptoms

• Black or brown spots on petals.
• Die back of soft stems.
• Die back from wounds caused by harvesting or pruning.

 

Botrytis on the stem

 

Botrytis on the flower petals

 

Control

• Do not harvest rose stems more than a centimetre above an eye (die back will occur if harvested too high above an eye).
• Do not prune if the growth medium is wet.
• Spray with fungicides.

 

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Bullheads

 

Bullheads are characterized by an increase in size of the flower and a lower than normal ratio of length to diameter of the flower, giving a flat-topped appearance to the bud instead of a pointed tip.

Bullheads are more prevalent during the winter when temperatures drop to levels lower than 12 degrees during early flower development.

Bullheads are associated with imbalances of gibberellin and cytokinin in the plant. Low temperatures reduce gibberellin activity and increase cytokinin activity. It is therefore important not to treat flowers during the winter with cytokinin containing root stimulants. In contrast treatment with gibberellin will reduce the number of bullheads.

TIP: Do not use cytokinin containing root stimulants during winter.

 

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Downy Mildew (Vals Meeldou)

 

Downy Mildew has caused severe losses in rose gardens.

Transmition

Transmitted when it is cool/cold and the humidity high (higher than 83%).

The mycelia develop inside the leaves and the spores are produced through stomata on the lower side of the leaf.

The optimal temperature for spore germination is 18 degree Celsius. Spores are killed by exposure to 27 degree Celsius for more than 24 hours. Sporangia germinates within four hours in water, and sporulation on leaf surfaces may occur in three days under ideal conditions. Spores may survive on dried fallen leaves for as long as one month.

Symptoms

• On the upper side of the leaves develop purplish red to dark brown/black irregular spots.
• Leafs may turn yellow.
• Leaf abscission may be severe.
• Under severe conditions small white conidia and conidiophores appear on the lower surface of the leaves.
• Under less favourable conditions spore production is sparse (hence the species name Peronospora sparsa) and difficult to detect.
• Purplish to back areas varying from small spots to areas 2 or more cm in length appears on the stems.
• Dead tips develop on calyxes and flowers are small and deformed.
• Stems are thin and the quality of growth is not good.

 

Control

• Remove infected material and destroy.
• Spray with preventative fungicides when the conditions are ideal, but remember that preventative fungicides will not kill the micelia inside the leaf.
• Be careful not the spread the spores with clothes from one plant to another.
• Do not irrigate in the afternoon when nights are cool.

 

 

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Red Spider Mite (Tetranychus urticae)

 

Visual symptom of Red Spider Mite damage:

General

Mites are not true insects but belong to the spider family. The best known of the mites is the Red Spider Mite. The presence of Red Spider Mite is one of the major disease related problems during summer in southern Africa. This pest is favoured by hot and dry conditions. They fed usually on the under side of leaves by means of sucking mouthparts. The eggs and mites are covered with a delicate web, which protects them from contact sprays. In recent years Red Spider Mite has become a major problem in our nurseries. For control of Red Spider Mite it is very important to understand the life cycle of the spider.

Life cycle

Egg – larva – nymph – adult
                                              

Up to 20 - 30 short cycles per year are possible under dry and hot conditions. Under ideal conditions this cycle can be as short as 8 - 12 days. Important is the fact that 75% of the adults are females and that one female can give rise to millions of spiders within a few months.

 

Control of Red Spider Mite during dry and hot conditions
(short cycle - high spider population)

When controlling Red Spider Mite the following must be taken into consideration:

• Red Spider Mites can become resistant to pesticides and pesticides must be varied frequently.
• Pesticides used for different parts of the life cycle must be used alternatively during different spraying sessions.
• Keep the humidity high during hot periods. (Wet the soil surface).
• Spray on the underside of the leaves and use small droplets for effective coverage.

 

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Powdery mildew

 

Powdery mildew is one of the most widely distributed and serious diseases in flowers and vegetables, both outdoors and in greenhouses.

In South Africa Powdery mildew is often associated with the dry hot summer months and during winter in heated greenhouses. This can easily be explained as follows:

Spore production, germination and infection take place when the humidity is high (90 – 99%) and the temperature cool (about 16 degree celcius).

Spore maturation, release and spread take place when the temperature is high (about 27 degree celcius) and the humidity low (40 – 70%).  Spores move through the air from one plant to another and air movement (wind) contributes to the spreading of the disease.

From this information it is evident that conditions during summer nights are optimum for spore production, germination and infection. During summer days the conditions are optimum for spore maturation, release and spreading. Several, repeated night-day cycles such as these would be necessary for the epidemic to develop.

Control of Powdery mildew in the garden is dependent upon the use of fungicides. Knowledge of the environmental conditions which favor the occurrence of Powdery mildew does allow for predicting the occurrence of the disease and, hence, for the use of preventative control measures.

 

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Stem cancer

 

Infection

Stem cancer gains entrance through wounds caused by pruning, harvesting, insects, thorns etc.

Symptoms

• Start as small yellow to red spots in the bark.
• The center of the cancer becomes light brown and the margin dark brown.
• The tissue dries out and shrinks.
• The plant parts above the cancer wilt and die.

Control

1. Harvest close to an eye.
2. Do not prune if the soil is wet.
3. Use sharp seccateurs.
4. Sterilize seccateurs.
5. Do not plant infected material.
6. Undercut infected parts and destroy.
7. Do not stress plants while storing or immediately after transplanting.
8. Spray wounds with fungicides used for “black spot”.

 

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Thrips

 

Symptoms

• Thrips are tiny insects (approximately 1 mm in length), of various colours (white, cream, yellow, brown or black).
• Their mouthparts pierce plant surfaces causing browning of petals and distortion of flowers.
• Leaves are distorted and generally curl upward. The leaves become crinkled and the laminae may be greatly reduced by high populations causing new leaves to be narrow.
• Damaging pods are distorted and show a network of russeted streaks caused by the thrips feeding injury.
• The laying of eggs into small developing fruit results in spotting of fruit, and the subsequent feeding of larvae causes scarring.

Thrips damage on petals

 

Thrips damage on leaves

Pest development

Survival of hibernating thrips is promoted by a mild winter, a sunny spring and abundant host plants and weeds in the greenhouse or in the areas surrounding the greenhouse. Female thrips lay colourless eggs in floral tissues. Adults feed on flower parts, later entering the soil to pupate. Thrip life cycles can range from 10 days to a month depending on temperature.

Control

Spray with a registered pesticides and remove weeds.

 

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Black spot

 

Black spot is the most important disease of garden roses all over the world.

Symptoms

Leaves:

Black circular or irregular spots 2 – 12 mm in diameter on upper leaf surfaces. The leaf tissue surrounding the spots turns yellow. Abscission of leaflets occur.

Stems:

Purple-red and irregular blotches develop on immature wood. Spots later become black.

Disease cycle

Leaves are most susceptible while still growing. Germination occurs only on wet leaves. The fungus does not survive in soil and over winters as mycelia in fallen leaves or in infected canes.

Control

Irrigation on the leaves should only be done on bright mornings with rising temperatures. Excessive watering should be avoided during dark, humid weather.

Removing leaves from the ground will reduce over wintering of the fungi. Dense planting should be avoided to allow good air circulation.

Fungicidal sprays should be used during periods of the year when conditions are favourable for black spot development.

 

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Nematodes

 

General

Nematodes are unsegmented roundworms. More than 15 000 species of nematodes have been described, and about 2200 species are parasitic to plants.

Although nematodes cause disease in many soil types, root damage is most serious on lighter, sandy soils.

Female rootknot nematodes feed on the roots causing galls to form whilst the eggs and larvae are present in the soil.

Plant-parasitic nematodes are a problem on roses worldwide.

The first above ground symptom is a general lack of vigor. Several symptoms may occur on roots, depending on the nematode specie and the number of feeding nematodes. Positive diagnosis of nematodes as the cause of observed symptoms requires laboratory identification.

Control

Nematodes have a very wide host range including many vegetable crops, flowers and weeds on which it can grow and over winter. It may be introduced into gardens via infected plants or through irrigation water.

Restricting the movement of infected plants and treatment with nematicides are effective disease management practices.

Nematodes on roots

 

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Crown gall

 

Crown gall is a bacteria and is transmitted by infected plants and via soil.

Control

1. Remove infected plants (remove from garden to prevent the distribution of the spores).
2. Plant disease-free plants.
3. Sterilise seccateurs.

Crown gall

 

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Herbicide damage

 

 

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Virus on leaves

 

 

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Aphids and white fly

 

Aphids and white fly are associated with the growth of a black fungi on the leaves.

Aphids

 

White fly

 

 

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