PEPPER - FUNGUS

Pathogen: Leveillula taurica (Lév.) Arn.

Host range: The fungus survives in plant debris, other cultivated host plants (tomato, aubergine, artichoke, etc.) and weeds.

Importance: Drought has significantly increased the importance of this disease on the field.

Powdery mildew overwinters as mycelium and spreads by conidia. It is a very widespread disease and easy to diagnose. It causes significant economic losses.

Symptoms: The most prominent symptoms are observed on the leaves (in greenhouse crops in particular), where necrotic light green or yellow lesions will form. In severe cases, the whole leaf will take a dull yellow or brownish colour. Necrotic spots form on the underside, where the wounds are located on the upper side. The spots are covered by white powdery mildew, a characteristic symptom of Leveillula taurica.

The disease spreads from older leaves to younger ones, causing defoliation of various severity. Plant and fruit development may stop altogether.

Disease cycle: The disease is spread by conidia, which produce a glassy, translucent mycelium when germinating. The mycelium penetrates leaf cells (parenchyma) through the stomata. Conidiophores emerge via the stomata, resulting in the characteristic “powdery mildew” symptoms. The disease typically infests pepper in hot, dry or humid regions. It develops optimally at 20-30°C and 70-80 % relative humidity.

Pathogen: Sclerotinia sclerotiorum (Lib.) de Bary

Host range: Sclerotinia sclerotiorum may infect a wide range of horticultural and arable crops and woody plants.

Importance: Sclerotinia is a common disease. Under optimum conditions it may kill the entire plant.

Symptoms: Sclerotinia causes soft rot on the stem. The mycelium appears as a white, cotton-like coating on infected stems, once the sclerotia have developed inside the stems. Stems shrivel and turn yellow, developing small watery spots which spread quickly, encircle the stem and turn into necrotic lesions. On these spots, white mycelia with black sclerotia may be observed. Sclerotinia overwinters in plant debris and soil in the form of sclerotia. Damage can be severe if the infestation attacks the neck or main stem of the plant. 

Disease cycle: Sclerotinia grows on or near the soil surface and can attack any part of the plant.

The disease starts from the sclerotia, which persist in the soil for up to three years.

The optimum temperature for the development of the disease is 21-25°C. High humidity and free moisture are essential for outbreaks. Once the plant is infected, the disease will develop even if temperatures drop.

Pathogen: Botryotinia fuckeliana (de Bary) Whetzel (Botrytis cinerea)

Host range: Botrytis can infect numerous horticultural and woody crops.

Importance: An important disease of pepper, able to threaten the viability of the crop by high infestation levels.

Symptoms: The pathogen is air borne or spreads by water and penetrates plants via injuries.
The most important symptom is the collapse of living tissue like leaves, young stems and flowers. On the leaves, soft necrosis lesions develop. Flowers are covered by grey mildew. The stems develop necrotic lesions. This necrosis hinders sap circulation, resulting in the wilting and eventual dying of tissues above the spot.
Fruits get infected through their stems or by contact with infected soil, flowers or leaves. Typical symptoms include usually soft, light brown circular spots, which may cover the entire fruit. On dead tissue, grey conidiophores appear.

Disease cycle: Botrytis cinerea can persist in soil and plant debris. The germination of spores is favoured by humid conditions (relative humidity > 80%) and by relatively cold temperatures. The optimal temperature range is between 18 and 23°C, although the fungus gets active above 0°C.

It may penetrate plants through (micro)injuries, but is also able to infect directly.

Treatment:
Preventive measures are recommended, as curative treatment does not work.

a) Chemical control:
Use systemic fungicides, combined with contact products in the case of severe infestations. Take care of avoiding injuries to the plant when harvesting.

As resistance may develop quickly, alternate fungicides.

b) Cultivation practices

• Avoid high humidity and low temperatures in greenhouse crops and use proper ventilation.
• Use a planting frame to facilitate the ventilation of the crop.
• Apply nitrogen fertilizers carefully.
• Treat pruning wounds (use herbal extracts to increase plant condition)
• Destroy infected plants and plant residue in infested areas.

 

 

ALTERNARIA FRUIT ROT

 

Pathogen: Alternaria alternata (Fr.) Keissler

Host range: Alternaria spp. is a pathogen of several horticultural crops.

Importance: The infection results in the internal rot of the fruit. The fungus may also cause secondary infections; in this case, black, rotten spots with concentric rings appear at the tip, spreading outwards. This fungus infects mostly by conidia. The spores are dark, long and multicellular. They divide easily and are airborne.

Symptoms: The fungus first attacks the placenta, seeds and internal surface of the fruit. The pathogen penetrates the fruit during flowering, via the pollen tube. If the pollen tube remains open after flowering, infection can occur at any time during the growing season. The cultivars most vulnerable to attack by the pathogen are the open style ones.

If the environmental conditions are optimal for pathogen development, spores may enter the plant through any opening or through the stigma or its remains. Infections start at flowering, with symptoms on infected fruits appearing as dark grey or necrotic spots on the inside of the ovary, the placenta and the seeds. The spots on the ovary are usually small, sunken lesions with a sharp edge and irregular shape. Seeds develop brown spots of various sizes. The placenta turns party black.

Disease cycle: The fungus persists as conidia or mycelium on plant residue. It is also transmitted by seeds and may even attack seedlings in the seedbed.
Alternaria spores are present in the air everywhere. To be able to penetrate fruits, they need high relative humidity which creates optimum conditions for germination and mycelium development. The optimum temperature range is between 12 and 20°C.

Control:

a) Chemical control:

• Sufficient Ca supply. Avoid excess nitrogen.
• Use systemic fungicides from flowering.
• Use seed dressing.

 b) Biological control:

Use antagonistic products, which contain competitive fungi to prevent its reproduction.

c) Cultivation practices

Avoid high temperatures and high relative humidity in greenhouses, as these promote epidemics. Grow less susceptible cultivars.

 

 

DAMPING OFF

Pathogen: Pythium ssp, Rhizoctonia ssp, Fusarium ssp, Verticilium ssp, Sclerotinia ssp.
Host range: These extreme polyphagous pathogens are present in the soil and seeds.
Importance: Pythium spp. are Oomycetes that may cause extremely severe damage in pepper in hotbeds. Rhizoctonia spp. (Basidiomycetes) occurs less frequently.
Symptoms: The main symptoms are the damping off and stunted growth of young plants.
Other symptoms include brown spots on the neck of young plants near the soil surface, which prevent water and nutrients from reaching the upper parts, killing off the plants. The root collar becomes thin and soft and seedlings die.
Disease cycle: Fungal activity is facilitated by factors such as the presence of organic matter, high humidity and low levels of lignification. The fungi also infect the soil.

Treatment
a) Chemical control:

• Apply fungicide seed treatment.
• Add fungicides to irrigation water in the hotbeds, immediately after seeding and on emergence.

b) Cultivation practices

• Ventilate hotbeds and avoid humid conditions.
• Use strong and heathy transplants with balanced root and shoot development.

c) biological control:

Use antagonists such as Trichoderma or Coniothyrium.

 

VERTICILLIUM WILT

 

Pathogen: Verticillium dahliae Kleb.
Host range: The main hosts are Capsicum frutescens (USA) and Capsicum annuum (EU). It also infests a high number of other cultivated species.
Distribution: 

Importance: Verticillium is soil-borne. The optimum soil temperature for infection is 12-13°C, by an air temperature of 19-23°C. It is a vascular disease, with the pathogen clogging plant vascular bundles. The optimum temperature for producing conidia is 25-28°C.

Verticillium causes severe outbreaks in hot areas. It rarely infects plants under moderate climates, with its damage limited to crops grown on dense soils.

Symptoms: The first symptoms appear as the withering and rolling up of old leaves. As the disease progresses, leaves turn yellow and drop. The pathogen invades phloem and disrupts sap transport which results in the darkening of bundles.
Unilateral wilt is a characteristic symptom, as the pathogen only infects some of the bundles.

Host range: The range of host plants is wide, including ornamental and horticultural species as well as wild plants.

Importance: The fungus belongs to Basidiomycetes and causes rot in pepper. Southern blight is most likely to occur in the summer months when the soil is warm and moist. It can occur in irrigated crops due to extreme weather conditions.

Symptoms: The fungus infects both field and greenhouse crops, attacking the stems near the soil surface. Symptoms include the discolouration of lower leaves, wilting and collapse, usually resulting in the death of the plant.

During outbreaks, plenty of white mycelium may be observed around the base of stems and roots, as well as in the surrounding soil. If the mycelium is visible with the naked eye, the best solution is to destroy the plant and the surrounding soil.

Embedded in the mycelium there are round, light reddish brown or dark brown sclerotia, similar in size to cabbage seeds. Infection causes wilting, which is usually the first symptom growers detect. Fruits and branches will only be infected when touching the soil. When infected late in the season, unilateral russeting may be observed on the fruits.