The leaves of shrubby Veronica species can be affected by leaf spot fungi (Stemphylium sp. and Septoria exotica), downy mildew (Peronospora grisea),

A white powdery fungal growth on the surface of leaves, buds, petals and shoots of roses and many other plants. The foliage may become discoloured, heavily infected young leaves may be curled and distorted, and the plant’s vigour may be reduced.

The leaves can be unsightly, covered in spots or distorted in shape, depending on the disease.  Premature leaf fall may occur in some cases.

Veronicas suffering from leaf diseases may show the following symptoms:

Leaf spot fungi

• Stemphylium sp. causes small, dark brown or purple spots on the leaves.  The spots do not merge together, nor enlarge in size.  The centre of the lesion can sometimes become pale in colour.  The infected leaves shrivel prematurely and can either remain on the plant or drop off.
• Septoria exotica causes spots that are more variable in size, generally being larger, with a tendency to merge or coalesce. The centre of the lesion becomes pale, with small, dark fungal fruiting bodies (pycnidia) embedded in the affected leaf tissues. The spots have distinct purple margins.  Some dieback of the shoots may also occur and the plants become unthrifty.

Downy mildew (Peronospora grisea)

• Distorted leaves are often the first sign of infection, especially with the larger-leaved cultivars. A grey/mauve felt-like 'fungal' growth appears on the under-surface of the leaves, with diffuse purple-red or yellow patches developing on the corresponding upper surface.
• In some circumstances a systemic shoot infection can develop, leading to leaf death and shoot die-back.

Powdery mildew (Podosphaera fuliginea)

• A white, dusty coating develops on the leaf surfaces.

Grey mould (Botrytis cinerea)

• The fungus can cause leaf death and shoot dieback, often accompanied by a grey-brown, 'fuzzy' fungal growth.

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

Similar non-chemical control measures can help to prevent infection by leaf spot fungi, downy mildew and grey mould, which are all 'wet weather' diseases (see the 'biology' section, below):

• Prolonged periods of leaf wetness should be avoided.  If watering is necessary, water to the base of the plant. Avoid overhead watering, but if this must be done then watering in the morning is preferable to the evening, as the leaf surfaces will dry more quickly.
• Providing good air circulation between plants allows the leaves to dry out quicker after rainfall.
• Whilst downy mildew can be a common problem for nursery-produced stock of certain cultivars, it is often less of a problem when plants are planted out in the garden, particularly if they are given good air circulation.
• Plants for sale should be examined carefully for any sign of downy mildew or leaf spots.
• Ensure that any leaves falling prematurely from an infected plant are collected up and disposed of (by burning, burial or consigning to the council green waste). Heavily-infected shoots could be trimmed off and disposed of in the same way.
• Choose less susceptible cultivars if downy mildew becomes a persistent problem. It is mainly a problem with broad-leaved cultivars such as Veronica ‘Midsummer Beauty’ and x ‘Franciscana Variegata’ but also occasionally present in some of the smaller-leaved species and cultivars, e.g. V. albicans, V. ‘Frozen Flame’, V. rakiensis, V. ‘Youngii’ (syn. ‘Carl Teschner’).
• V. ochracea ‘James Stirling’ appears to have some natural tolerance to the disease.
• V. 'Green Glow' is one of the more susceptible cultivars to grey mould.

Prolonged leaf wetness is not required for infection by powdery mildew, but high humidity is, so providing good air circulation is still important. Avoid drought stress and the overuse of nitrogenous fertilisers.

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

There are no fungicides available to amateur gardeners with activity against downy mildew or grey mould.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

Further information on the management of downy mildews, powdery mildews and grey mould can be found in the advice profiles on the links below:

Downy mildews
Powdery mildews
Grey mould

The primary means of spread between plants by the leaf spot fungi is thought to be via asexual spores (conidia) produced on infected leaves.  The disease spreads to adjacent plants through direct contact of the leaves or through the asexual spores spreading/dispersing in rainwater or on the wind. Prolonged leaf wetness is necessary for spore germination and infection.

Under certain environmental conditions some Stemphylium species are thought to be able to produce a sexual stage, and spores liberated from the fruiting bodies of the fungus in this scenario could account for the overwintering survival of the fungus and its long distance dissemination.

Downy mildew requires cool wet weather for infection and spread. The 'fungal' growth seen on the underside of the leaves produces large numbers of asexual spores. Downy mildews are host-specific, so for example the downy mildew affecting a Veronica will not infect a rose or vice versa.

Asexual spores (conidia) of the grey mould fungus are always present in the air. They require leaf wetness or very high humidity to germinate and infect. Damaged or senescent plant parts are a very common way in which the fungus gains entry to the plant. The disease has a huge host range, and is active over a wide temperature range.

The conidia of powdery mildews have an unusually high water content, enabling them to infect under drier conditions than most other fungal pathogens. High humidity is, however, favourable for spore production and infection. Powdery mildews are also host-specific.