White powdery clematis leaves?

It's probably a slight case of Powdery Mildew but until it can be physically seen close up it's not a certainty, but this hot/warm and very humid weather is just what fungi like so the odds on are that it is.

RHS advice.....

"What is powdery mildew?
Powdery mildew is a fungal disease of the foliage, stems and occasionally flowers and fruit where a superficial fungal growth covers the surface of the plant.

Many common edible and ornamental garden plants are affected including apples, blackcurrants, gooseberries, grapes, crucifers, courgettes, marrows, cucumbers, peas, grasses (the powdery mildew fungi are major pathogens of cereal crops), Acanthus, delphiniums, phlox, many ornamentals in the daisy family, Lonicera (honeysuckle), rhododendrons and azaleas, roses and Quercus robur (English oak).

Powdery mildews usually have narrow host ranges comprising of just a few related plants. For example, the powdery mildew affecting peas is a different species from the one attacking apples.



Symptoms
You may see the following symptoms:

• White, powdery spreading patches of fungus on upper or lower leaf surfaces, flowers and fruit
• Tissues sometimes become stunted or distorted, such as leaves affected by rose powdery mildew
• In many cases the infected tissues show little reaction to infection in the early stages, but in a few specific cases, for example on Rhamnus, the infection provokes a strong colour change in the infected parts, which turn a purple-brown colour
• Sometimes the fungal growth is sparse and difficult to see, despite discolouration of the plant tissues, e.g. on the undersurface of rhododendron leaves

Control


Non-chemical control
Destroying fallen infected leaves in autumn will reduce the amount of infectious spores next spring. Promptly pruning out infected shoots will reduce subsequent infection.

Most powdery mildew fungi have a host range restricted to a relatively few, related plants, but these can include wild relatives which can be sources of infection, e.g. wild crab apples may be sources of infection for apple orchards.

Seed producers sometimes offer powdery mildew-resistant cultivars of both vegetables and ornamental plants, check catalogues for details.

Manage the environment around susceptible plants:

• Choose the best location
  • Plant in full sun where possible (the exception is shade-loving plants)
  • Avoid sheltered places such as against walls or fences where the air is likely to be still
• Manage the soil
  • Provide good drainage, but water regularly during dry periods (plants suffering from regular but intermittent drought-stress are more susceptible). Application of mulch may help water retention
  • Do not over-fertilise (especially not too much nitrogen) as succulent growth is particularly susceptible
• Manage humidity and leaf wetness
  • Don’t plant too densely, giving space between plants. Aim for an open environment to increase air circulation and reduce humidity
  • Prune shrubs to give an open structure (which will again improve air circulation and reduce humidity)
  • Avoid overhead watering early in the morning or in the evening
  • Overhead watering (in mid-morning) in dry weather can reduce powdery mildew (which does not like direct contact with water). Avoid leaves remaining wet for long as this may favour other diseases

Chemical control
Because most of the growth of powdery mildews is found on the plant surface they are easily targeted with fungicides.

Edibles: Tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus) can be used on glasshouse crops of cucumber, courgette and summer squash.

Ornamentals: The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are approved for the control of powdery mildews on ornamental plants.

The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Resolva Rose 3 in 1, Rose Shield Bug & Fungus Killer, Roseclear Ultra Gun 2, Rosegarde) and triticonazole containing acetamiprid (Roseclear Ultra, Roseclear Ultra Gun).When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.

Any crop: SB Plant Invigorator, Resolva Natural Power Bug and Mildew Control, RHS Bug and Mildew Control, Ecofective Bug & Mildew Control and the Ecofective ‘Defender’ range contain a blend of surfactants and nutrients and can be used on any edible or ornamental plants, with no harvest interval. They have a physical mode of action and may be used against powdery mildews, as well as a range of pests such as whiteflies, aphids, spider mites, mealybugs, scale insects and psyllids.