Common Diseases of Conifers in Oklahoma - OSU Fact Sheets ...

Oklahoma Cooperative Extension ServiceEPP-7618Common Diseases of Conifersin OklahomaKenneth E. ConwayResearch Plant PathologistBrian OlsonPlant Disease DiagnosticianOklahoma Cooperative Extension Fact Sheetsare also available on our website at:http://osufacts.okstate.eduFoliage diseases of conifers can be of great importanceto the survival and general vigor of these trees in Oklahoma.The green needles of conifers often represent several yearsof growth and the tree depends upon all of these needles forits photosynthetic needs.Conifers will remain green throughout the year by graduallyshedding the older inner needles and at the same timeproducing new needles from the tips of the branches. Unlikebroadleaf, deciduous trees that completely replace only aportion of their needles. Complete defoliation of conifers isoften fatal. Even partial defoliation can result in unhealthy,disfigured trees because appreciable loss of needles is notfollowed by a complete replenishment of the following year.Conifer diseases are of most economic importance tohomeowners due to the off-color of infected foliage and thepoor appearance of the tree. In order to assist homeownersand commercial growers to understand several of the moreimportant conifer diseases that occur in Oklahoma and theircontrols, the following diseases will be discussed: DothistromaNeedle Blight, Diplodia Tip Blight, Needle Casts, PhomopsisBlight of Junipers, Cedar Apple Rust, and Pinewood NematodeDisease.Dothistroma Needle Blight of PinesDothistroma Needle Blight can cause the prematureloss of needles of several varieties of pine. In Oklahoma,Dothistroma Needle Blight has been recorded from Austrian,Ponderosa and Mugho pines. This disease, if uncontrolled forseveral years, can severely weaken and eventually kill thesetrees.SymptomsSymptoms develop in the fall of the year in which theinfection was initiated. Early symptoms consist of yellowand tan spots, and water-soaked bands on the needles. Thebands and spots may turn brown to reddish brown and maybe surrounded by yellow bands. the tips of the needles willdie, with the base of the needles remaining green. The deadportion of the needle may break off leaving a blunted tip.Extensive damage may occur on the needles within two tothree weeks of the initial appearance of symptoms. Infectedneedles will be shed or cast from the tree prematurely. UnderOklahoma conditions, needle fall is extensive following hot,dry summers. (Figure 1.)Disease CycleThe nonsexual stage of the fungus Dothistroma pini isthe only stage found in Oklahoma. Conidia (spores) of thefungus are produced in stromata (a mass of fungal myceliumin or on which spores are formed) in the spots and bandson the needles. The stromata begin to form in the fall of theyear, but most mature to produce conidia during the followingspring. Conidia are spread by rainsplash throughout thegrowing season (May - October). Infection occurs throughoutthe growing season, but symptoms do not develop until latesummer or early fall. Two seasons are required for completionof the disease cycle.ControlCopper fungicides (Bordeaux mixture 8-8-100, Tenn-Cop5E, or Copper Fungicide 4E) effectively control DothistromaNeedle Blight. Two applications are recommended,the first in mid-May to protect the previous seasons’ needlesand the second in mid-June to July to protect the current-yearneedles. In addition, homeowners can help control this diseaseby collecting the infected needles and destroying them.Diplodia Tip BlightSymptomsThis fungal disease can seriously attack pine seedlingsin nurseries, causing a rot that starts below the soil line in thecollar area of the stem and extends upward into the main stem.The scales of young cones are also attacked. This diseasealso causes a dieback of the branches of older pine trees.Growth from these blighted terminals is usually stunted, theneedles turn brown, and the terminal buds exude an excessiveamount of resin (Figure 2). Diplodia can also infect the conesof these older pines and the minute black fruiting bodies caneasily be seen on the scales of the cones (Figure 3).Disease CycleDiplodia pinea infects the plants through the youngneedles. Infection takes place through small openings in theneedles called stomates. Infection can also occur throughwounds and cracks in the bark.Division of Agricultural Sciences and Natural Resources • Oklahoma State University

Figure 1. Dothistroma needle blight on Austrian pine.ControlSeedlings and young trees that are infected with the stemrot phase cannot be treated successfully. When Diplodia TipBlight has been a problem in seedling production, the use ofsteam-pasteurized soil or soil fumigants will help in controllingroot and stem infection.Diplodia Tip Blight on older trees can be controlled bypruning and sanitation. As soon as blighted terminals andcones are noticed, the needles, twigs and cones should bepruned to healthy tissue and destroyed. Do not prune whenthe branches are wet because the conidia of the fungus caneasily be spread when moisture is present. Where infectionhas been severe, the use of Bordeaux mixture or CopperFungicide 4E will control this disease. It should be appliedearly in the spring, when the buds open, and twice more atweekly intervals until the needles break through the needlesheaths. An application of fungicide in the fall may also aidin slowing the spread of the disease; however, fertilizationand watering in the fall may be more beneficial. Homeownerswho maintain vigorous trees through good tree health carewill have fewer problems with Diplodia Tip Blight.Needle CastsSymptomsThe first indication of this tree disease is the scorchedbrown or yellowish appearance of most of the needles. Manypines are affected by this disease and it can become a severeproblem in Christmas tree plantings. The initial discoloration ofthe needles is followed by needle fall or casting. Few trees areactually killed by this defoliation but sustained needle kill willeventually reduce the overall growth rate and make the treemore susceptible to other diseases and insect attack. Over40 different species of fungi are capable of causing needlecase. The more important ones are Lophodermium pinastri,Hypoderma lethale, and Hypoderma hedgcockii.Disease CycleSmall black elliptical fruiting bodies of the fungus areproduced on the needles. These structures break open in thelate spring or early summer during periods of wet weather andrelease great numbers of spores. The spores can be spreadby splashing rain or wind to infect other needles or trees.Figure 2. Stunted terminal branch of pine infected withDiplodia pinea.Infected needles begin to turn brown during the late fall andearly spring and will have a mottled appearance due to thedeath of needle tissue at the infection sites. Temperature andhumidity are very important in determining the severity of theinfection.ControlIn nurseries this disease can be controlled by the applicationof the fungicide after the needles are half grown followedby a second application when the needles are fully developed.No chemical control measures are normally recommendedfor homeowners even though the tree may appear unsightly.Collection of infected needles and their destruction will greatlyreduce this disease. However, if a homeowner wishes tocontrol this disease, the proper application of a fungicide willprovide excellent control.Phomopsis Blight of JunipersThis is the most common disease of cedar, juniper, andarborvitae in Oklahoma. Phomopsis Blight can be a severeproblem in nursery plantings and can cause the death of manyseedlings. Older trees are less affected by Phomopsis Blightand unless these trees are environmentally stressed or inpoor health they are usually not affected enough to warrantchemical sprays.Symptoms and Disease CycleInfected trees will exhibit browning of the foliage and dyingof twigs and branches (Figure 4). Small black fruiting bodies ofthe fungus, Phomopsis juniperovora, are formed on this deadtissue. Conidia are produced in these structures and whenmoisture is present masses of conidia will ooze out to formtendrils or horns. The conidia are spread by rain and variousother means such as mechanical disruption and insects. Thefungus enters the plant through wounds as well as unbrokentissue. The browning of the needles and twigs begins at thetips and progresses downward toward the stem. Moisture isan important factor in disease spread and severity. Nurseriesand Christmas tree plantations that utilize overhead irrigation7618-2