Biology of Mycosphaerella

This information was last updated in 2015.

Understand the biology and epidemiology of Mycosphaerella and how it can spread.

Survival and spread

The fungus survives in/on crop residues. This includes infected tendrils on crop wires, decaying plant debris in soil, and seeds.

Survival can be more than a year in dry crop residues.

At temperatures below 5 ̊C, the fungus hibernates and can then survive temperatures below freezing.

Epidemics

In glasshouses, the fungus also survives on the glass, concrete, aluminium, etc.

It can survive on seed which leads to early infection of the radicle, the hypocotyl and the embryo.

However, such seed infection is not significant in the epidemics in the UK as seed is routinely cleaned.

No seed-borne infection was found in random seed-lots tested as part of AHDB project PE 001.

First infection

First infection usually occurs from airborne ascospores produced by the surviving fungus (either inside or outside the glasshouse) when moisture is available for fungal growth.

The wind-dispersed ascospores are blown into the crop. They then germinate when they reach the plant and infection conditions are favourable.

On infected plant tissue, new ascospores as well as conidia (vegetative spores) are produced.

Downward spread

The fungus differs from most other fungi as the ascospores are produced throughout the year.

The conidia are dispersed by water, mostly downward.

Both types of spores can also be spread by tools, machinery, and clothing.

Infection conditions

When temperatures are above 5 ̊C, the fungus produces ascospores which are spread by air currents (wind) into the new crop.

Release of ascospores requires a high (>85%) RH and 1-10 hours of leaf wetness.

There is a to ascospore release, with peaks in discharge occurring in the evening. This is the case especially in the three hours after sunset and early morning due to the changes in RH.

At sunset, this coincides with optimum infection conditions of high RH.

Spore germination

Ascospores germinate and produce germ tubes which grow on the surface of the plant before infecting the host tissue. In this phase, the fungus needs moisture and nutrients.

Germ tubes grow faster on older or damaged tissue because the plant leaks more nutrients there.

Germ tubes stop growing under dry conditions. When temperatures are below 25 ̊C, however, growth restarts when the RH increases again.

Penetration of the plant

Penetration of the plant (infection) by the germ tube is either directly through the epidermis or stomata, or in wounds.

It can take place at the stem, on leaves, and on flowers. If penetration occus on the flowers, this leads to internal fruit rot and black rot can occur if penetration takes place directly on wounded fruits.

After penetration of the plant, the supporting tissue (parenchyma) is infected and destroyed.

On infected tissue, new conidia and ascospores are formed which spread to neighbouring plants or plant parts.

At the base

Early infection of stems often occurs at the base of the plant, though this is often overlooked.

Higher up the stem, stubs resulting from pruning and harvesting activity form entry wounds. It is common to see infection sites spreading up the main stem of the plant via such hands-on operations.

The wounds supply the fungus with sufficient moisture and nutrients for infection.

Stem infection

The stem is also infected by growth of the fungus from infected leaves through the petiole into the stem.

Such stem wounds can develop in two ways:

• Aggressive black lesions quickly form which girdle the stem and kill the plant
• More superficial silvery lesions form

The latter, apart from acting as a further inoculum source, doesn’t seem to cause significant damage to the plant itself.

Aggressive or superficial

It is not clear at this stage what differentiates the aggressive and superficial Mycosphaerella lesions and further investigation is required.

At the growing tip, a high RH is required for infection. Serious infection occurs at RH above 95% (HD <1.0). Dew formation is not required.

There is some evidence from AHDB project PE 001 to suggest that vascular ‘systemic’ infection may occur via this route. This creates weak lateral shoots, but further investigation would also be required to confirm this.

Leaf infection takes place mostly on damaged leaves and petioles, and via the hydathodes (the end of veins at the leaf margins). In this case, enough moisture and nutrients are available.

Stimulated by moisture

Guttation from leaves and dew formation greatly increases infection as spore germination is stimulated by the moisture.

Guttation from cucumber leaf hydothodes

The susceptibility of the leaves decreases with increasing light due to surface drying of leaves and increased strength of leaf tissue.

Cultivars and cropping systems

No resistance genes have been found, so there are no resistant or tolerant cultivars.

The shape and size of the flowers may affect susceptibility. Although no straightforward correlations have been found, cultivars with smaller flowers are preferred.

Also, cultivars that drop their flowers more readily are likely to be less susceptible to Mycosphaerella infection.

More generative cultivars have a more open leaf architecture and lower root pressure. They are less susceptible than the more vegetative cultivars.

Modern cultivars

Modern cultivars with some tolerance to powdery mildew tend to have better developed root systems and vegetative growth. These appear to be more susceptible to stem infection.

In addition, these cultivars are sprayed less frequently with fungicides that could have a side effect against Mycosphaerella. ,This further increases the problem in these cultivars.

Infection by Mycosphaerella is greater in high-wire cropping systems, probably because of the higher root pressure in the plants which increases guttation.

This effect can be counteracted by maintaining an open crop to reduce humidity within the leaf canopy.

Interaction with other pests and diseases

Pests can create entry ports for Mycosphaerella in flowers and on leaves.

Mycosphaerella may occur together with grey mould (Botrytis cinerea) on stems.

Both fungi benefit from nutrients and moisture leaking from wounds on the stem. Damage of the flower or leaf tissue caused by pests also increases both diseases.

Powdery mildew

Mycosphaerella is inhibited, to some extent, by most fungicides used against powdery mildew. However, the level of suppression provided is insufficient to prevent epidemic development in most seasons.

A heavy powdery mildew infection can damage the leaves, so Mycosphaerella can infect them more easily. When heavy powdery mildew infection leads to more fungicide application, the disease usually decreases.

Authors

This information was originally authored by Aleid Dik and Martin McPherson