First some old enemies: Volutella buxi, or stem blight, is an opportunistic disease prompted by injury, where it enters through wounds. It is characterized by separate dead branches and leaves with pink spots underneath that remain on the branch. Best prevention is good air circulation, no sprinklers, and no excess fertilization. You can cure this one by pruning below the infected area with bleach-dipped pruners and using fungicides alternating every 2 weeks. Mulch helps prevent the disease by damping down infected water splashing up on leaves. Decomposed granite used as a mulch has the added virtue of raising pH. Use bleach on shoes, clothes and vehicles. Suffruticosa is most susceptible.
Boxwood leafminer, little white lines on the underside of leaves, requires a spraying protocol—when dormant use horticultural oil, in spring and fall use soil systemics and in late May use Tri-Star (not so bad for honeybees). Summer foliar treatments are Orthene and oil or Avid and oil. Saunders Brothers handed out information on their leafminer trials of the past several years, which is appended to these notes.
And the bad boy: Cylindrocladium pseudonaviculata, aka boxwood blight, scourge of Europe, land of a million boxwoods. Introduced into the US only in the past few years, the blight thrives in mild and rainy conditions and travels by touch, not by air. The spores are sticky and catch onto passersby, animals, tools and debris. It shows up as round spots encircled by yellow rings. The leaves soon fall off the stems leavings patches of defoliated branches. Sweetbox and pachysandra are carriers. Best practices include creating good air circulation through judicious pruning, mulching, purchasing from reputable dealers and quarantining plants for 90 days before planting. Keep discarded holiday boxwood displays away from live boxwood. Little can be done once the blight has struck although you can try by removing all infected foliage and cutting stumps to no less than 10”. Dig up and dispose of infected plants and foliage in plastic bags. Growers are working hard to find/create more resistant varieties. Green Velvet, Green Beauty, Buxus sinica var. insularis ‘Nana’ are fairly resistant. De Runk, Fastigiata, Suffruticosa are not.
Bottom line: Many of these problems are aggravated by moisture. Prune when dry to open up the branching, mulch to prevent splashing up from below, don’t use spray irrigation, and select resistant varieties from reliable growers. Varieties resistant to some diseases are not always resistant to others.
Information on these diseases and more can be found at www.buxuscare.com and the very helpful Sauders Brothers Boxwood Guide, 5th and latest edition at www.SaundersBrothers.com.