We try to discuss plant diseases before they occur to help you prepare for disease management. With the early occurrence of many diseases in Illinois this year, we may be a bit late from some areas of the state. Juniper blight may be caused by two closely related fungi, Phomopsis or Kabatina. It is important to understand the difference because the former can be managed quite well, but we have fewer choices with the latter.
Juniper tip blight, also known as Phomopsis blight, is caused by a Phomopsis fungal species. It is the most common disease of junipers in the Midwest. Most damage occurs on eastern red cedar and on creeping, Rocky Mountain, and savin junipers, but we have seen the fungus on arborvitae, Douglas fir, fir, yew, and larch, as well as junipers. The newest growth is susceptible to infection and becomes resistant once needles become a normal, dark green. The growth that is now emerging is susceptible to this fungus.
Plant growth is ahead of schedule this year, so some areas of the state may have already passed through the susceptible growth stage. Phomopsis infection occurs on the youngest needles, starting as yellow spots. Shoot tips then turn light green before becoming brown. Homeowners usually dismiss early symptoms as winter burn and do not become concerned until the appearance of brown shoot tips. One diagnostic feature to help identify this disease and distinguish it from weather scorch is the presence of a grayish band at the base of the dead shoot. In this band are pinhead-sized black fruiting bodies (pycnidia) of the fungus. The pycnidia are visible with the naked eye or with the aid of a hand lens. If the tissue is very dry, place it in a plastic bag with wet paper towels overnight. The fruiting bodies will be easy to see the next day.
Infection by Phomopsis can occur when succulent new growth is present in wet weather. The fungus is also very persistent. Spores germinate under moderate temperatures (60°F to 82°F) and high humidity within 7 hours after coming into contact with the new foliage. If the foliage dries before infection occurs, the spores are not killed; they begin growth again with wet weather. Pycnidia form 3 to 4 weeks after infection. Spores are dispersed by splashing rain. Watch for this disease soon.
Phomopsis blight may be controlled by pruning and removing infected foliage when the plant is dry and by using preventive fungicides. If you are willing to start your planting over from scratch, use resistant varieties for the easiest long-term control. If replanting is not an option, then pruning is important because the most common source of the fungus is infection from the previous year. Prune only dry foliage to avoid spreading spores and to lessen the risk of infection by other fungi. Fungicide recommendations are provided in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook 2000, as well as the Illinois Homeowners’ Guide to Pest Management. Report on Plant Diseases No. 622 contains more details about Phomopsis blight. This report is available on the Extension Web site at http://www.ag.uiuc.edu/~vista/horticul.htm.
Kabatina blight is the other juniper blight. It is caused by a fungus that will appear very similar to Phomopsis unless you have the capability to look at fruiting bodies with a microscope. The significant difference in these diseases is the time of symptom development. Phomopsis blight will occur on new growth, with infection occurring this spring. Kabatina blight occurs on last year’s needles. You might see it on your junipers in March or April on what you believe is the new growth. That is actually last year’s growth. This year’s growth is much lighter in color and emerges in May and June.
The other significant difference in these diseases is management. Kabatina blight is not clearly understood, and fungicide timing has not been effective in disease control. It is important to remove and destroy infected twigs in dry weather. Infection is thought to occur through wounds the previous season. Reports indicate that disease-resistant varieties are in development, so ask for these at your nursery. Some information about Kabatina blight can be found in Sinclair, Lyon, and Johnson’s book, Diseases of Trees and Shrubs. A very good Web reference put together by the University of Nebraska can be found at http://www.ianr.unl.edu/PUBS/plantdisease. Both of these juniper blights are discussed and pictures provided.