Derived from Earthworm Castings - Root Growth
Horticulture Hotline 8/21/23
By Bill Lamson-Scribner
Boxwoods are under attack in the Lowcountry by so many different blights, root rots, declines, cankers, insects, mites, nematodes, and diseases it is hard to keep track of anymore. If you are having trouble with your boxwoods, you should consider taking a whole plant (roots included - Phytophthora) and some neighboring soil (for a nematode test) to Clemson Extension to figure out exactly what is going on so you know how to remedy the situation. Once you collect the samples head straight over there. Nematodes are like dogs and babies, they do not like sitting in a hot car. Probably best to make your drop off on Monday, Tuesday, or Wednesday early, so your samples will not sit in a hot USPS warehouse over the weekend. Consider Yaupon Hollies as a replacement.
Something I wrote up for a private plantation five years ago – products may have changed, but it shows the widespread issues.
Boxwoods 10/1/2018:
Must prune to get more sun and air movement in that area. When pruning, disinfect pruning shears frequently in household bleach diluted 1:9 with water or rubbing (isopropyl) alcohol for 10 seconds. You do not seem to have an issue in other areas of boxwoods with more sun and air movement. Rake up any fallen leaves for sanitation. The fungus Macrophoma is a foliar pathogen that causes the oldest leaves to turn tan; small black fruiting structures can be seen on the killed leaves. Shake the major branches of your boxwood one by one to dislodge the infected leaves and then rake them from under your boxwood. Spray Lime /Sulfur spray on the ground only after you rake. Dispose of them so the spores do not cause new infections.
Another fungus, Volutella, may cause the branches to develop the color progression from orange to tan. Look for orange-pink fruiting structures along the branches in wet, warm weather and prune out infected branches. Macrophoma and Volutella are most destructive to boxwood varieties with a tight, compact habit or to boxwood that have been sheared repeatedly. The tight foliage results in poor air circulation and slow drying after rain or dew (try to water the ground - not overhead irrigation); fungal diseases thrive in these dank conditions.
The best way to control these diseases is to improve air circulation around the plant and within the plant by thinning the branches or adding fans. Cut some of the small branches back by about six inches. Thin the plant enough so you can begin to see the overall branch structure of the shrub. Avoid shearing since it promotes compact, twiggy growth (shell effect) and injures leaves, making them unsightly.
Phytophthora Root Rot - Soil Surface Spray: Apply to soil surface in a broadcast or banded spray in sufficient water to obtain thorough coverage of the plant root zone. For best efficacy, irrigate with at least 1/2 inch of water within 24 hours. Use Regulate at 2.5 oz/M every 10 weeks.
Foliage Spray: Apply Reliant (you have in barn) as a foliar spray at 1 tablespoon/gallon every 14 days.
Mixed with Fish / Seaweed Blend at 2 tablespoons/gallon every 14 days.
Spray for Volutella Blight:
Honor Guard or Propiconazole – 1 oz / 25 gal every 14 days mixed with Pentathlon LF 12.5 oz/25 gal
Rotate after one month with CuPro 5000 at 1.5 tablespoons / gallon
Nematodes – Crab Shell – 30#/M. Use products that encourage rooting. VermaPlex(derived from worm castings), SeaHume, SuperTHRIVE are a few.
Other cultural practices to help them out as much as possible:
Make sure they are getting proper amount of water – too much water or too little will injure them (dig down in the soil and check moisture). Since many boxwoods are in formal settings around brick work, make sure restricted area has soil for the plants and is not filled with roots. ‘Pot -bound’ if you will. Possum’s Wetting Agent with Biostimulants will help watering the area and help grow roots. Water the ground, not the foliage.
Control mites, leaf miners and other pest that could weaken the plants or act as a disease vector.
Soil Test Possum’s – when we get results – see where you are. Correct fertilizer and pH are very important for optimal plant growth.
Nematode test Clemson.
T-Methyl and Chlorothalonil are good rotation products for certain diseases to avoid resistance.
Always read and follow product label.
