Boxwood blight is a fungal disease caused by Calonectria pseudonaviculata ,  and C. henricotiae

Boxwood blight infects all aboveground portions of the shrub but not its roots. Symptoms begin as dark leaf spots that form brown blotches. A key symptom that differentiates boxwood blight from other boxwood diseases is that narrow black streaks (cankers) develop on green stems. White spores exist on undersides of infected leaves and on stem cankers, spores add to spreading the disease. This disease defoliates the shrub usually from the ground up. Severe defoliation and stem dieback can kill young plants and causes older plants to look unsightly losing ornamental value. This blight cannot be cured once it has started.

It’s possible plants carrying the fungal pathogen won’t show symptoms of the disease if the infection is very recent. The fungus that causes boxwood blight can overwinter on infected plants and in infected leaf litter. Spores of fungus can spread from rainfall or irrigation. It can spread within one plant or to nearby plants. It can also spread larger distances by infected nursery stock or contaminated landscape tools.

If boxwood blight doesn’t kill the boxwood it will still weaken the plant so much so that another pathogen could easily kill the boxwood instead. It is the best practice to destroy all infected boxwoods by a controlled fire burn or burying all infected plants away from contact of any new boxwoods as this disease spreads so rapidly.

The only way to prevent further spreading once disease is established is by practicing extremely thorough sanitation practices. You must disinfect pruners and tools frequently and between different plant usage. Never trim or prune boxwoods when they are wet or after heavy rainfall. Wash boots/shoes in between house visits, landscape installs, garden center visits. Mulch should theoretically reduce disease development by reducing pathogen dispersal via rain splash – a primary cause of boxwood blight’s spread.

Preventatively spray a fungicide. Infuse and Fungonil by Bonide can help prevent the disease with repeated applications.

The best resistant boxwood  cultivars are: Green Beauty, Green Gem, and Green Mountain

The best option is to choose a different plant entirely and do not plant boxwoods. Other evergreen foundation plant options include Japanese Holly, Inkberry Holly, Plum Yew, Japanese Skimmia, Andromeda, Otto Luyken Cherry Laurel, Skip Laurel, and others depending on the application.  We are also testing other new varieties which hold promise for certain plantings.

What is leaf miner: Leafminer is an insect pest in which the larvae feed on the inside of the boxwoods leaves. These insects feed inside the leaf between the upper and lower leaf surface. Larvae will turn into adults and break through the lower leaf surface when fully matured.

What to look for: Blistered leaves, leaves with something resembling a “bubble” protruding out on the leaf surface, leaves may be discolored brown to yellow or they can remain green.

Life cycle of leaf miner: Larvae stage causes the most damage to boxwood plants. Larvae over winter in leaf blisters. Once spring arrives with warmer temperatures the larvae use this as a signal to become active again and start feeding inside the boxwood leaves. Larvae can feed inside the boxwood leaves from spring to summer. Adult leafminers can emerge out of the leaf in May. Adults look like small yellow/orange flies that can swarm around boxwood plantings. Shortly after adults emerge they mate. After mating a fertile female can deposit her eggs into the underside of boxwood leaves directly injecting the leafminer eggs into the boxwood leaves. Adult females die shortly after depositing their eggs. This starts the cycle again as the eggs grow into larvae within the leaf in about two weeks. The larvae can feed and grow to maturity inside the boxwood leaf bringing a lot of damage to the boxwood plant as it feeds and steals nutrients from the plant itself.

How to treat Leaf miner with insecticides & systemics: Use insecticide around the first week of May and a second application around mid June and mid July. Carbaryl or Malathion can control adult populations. Acephate can control larvae developing in the leaves when applied in mid May after adults emerge from leaves. You can also use the systemic insecticide imidacloprid also known as Merit to control larval populations. Apply systemic insecticide at the base of the plant near roots because this insecticide will be taken up through the roots of the boxwood and concentrated inside the boxwood to prevent further infestation.

How to treat Leaf miner naturally: Select more resistant cultivars to begin with, English and Japanese varieties are better than American varieties. Use natural insect predators such as green lacewing to control larval population. You can prune new growth of Boxwoods in May after adults deposit eggs to reduce the larval population and reduce damage on your boxwoods. Keep Boxwoods healthy, with proper watering and air circulation. A healthy plant is less likely to be susceptible to insect damage.