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I have some small box-trees which start to have some yellow leaves on the top of some branches. Is this the start of box-blight or just the effects of winter. We have some low temperatures (down to -5 C) here and snow (20-30cm).

Please advise with the help of the pictures below. (Click to enlarge)

                             

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This looks like winter burn to me based on:

  • absence of circular necrotic spots on the leaves
  • boxwood blight (Cylindrocladium buxicola (syn C. pseudonaviculatum))should be less active during winter months as it is usually drier and colder
  • absence of dark or blight on the stems

From here winter burn is described as:

Winter damage occurs on boxwoods (Buxus sp.) when unseasonably warm winter days are followed by freezing temperatures. During the warm period, the plant begins to come out of dormancy and the cambium or conductive tissue begins to fill with additional water. When the temperature drops below freezing, the water freezes. The expanding ice splits the tissue resulting in death of the affected tissue. Cold, dry winds pull moisture from the affected branch resulting in a freeze-dried, "freezer burn" effect. Depending upon conditions, whole plants, just sections of a plant or only the tips of branches may be affected

Boxwood blight is described in detail here.

  • It is considered to be endemic in Europe and other areas of the world.
  • Warm humid weather with abundant leaf litter to act as a source of infection promote the growth of the disease
  • it can attack cuttings, containerized plants or well established plants
  • fungal controls only give some relief

As is typical with many fungal diseases keeping the ground clear of debris, using a mulch to reduce soil splashing on leaves and good cultural practices will go some ways to preventing this.

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