Sustainable Management of Common Plant Diseases in the

LandscapeDr. Elizabeth Little

Department of Plant Pathology

University of Georgia

Plant Disease TrianglePlant Disease Triangle

Host Pathogen

Environment

Disease

Stressed or injured plant

Capable of causing disease (many are host specific)

WATER! (wet foliage or soils, high humidity, poor air circulation)

Types of Plant Diseases

• Root and crown problems– Root rots, crown rots, nematodes, galls,

drought, overwatering, poor planting, poor soil, inappropriate site

• Foliage and stem diseases– Leaf spots, cankers, herbicide damage

• Systemic diseases– Viruses, bacterial scorch, aster yellows

Soil and Fertility Management• Healthy plants resist disease• Proper site preparation will avoid

many problems.• Right plant/right place• Promote healthy plants and healthy

soils with:– Compost amendments– Mulching– Nutrient additions

• Most plant health Most plant health problems in the problems in the landscape are due to a landscape are due to a problem with the roots problem with the roots or soil, but diagnosis of or soil, but diagnosis of root problems can be root problems can be confusingconfusing

• The above ground The above ground symptoms of any root symptoms of any root stress will look the stress will look the same, even though root same, even though root stress can have stress can have different causes: poor different causes: poor nutrition, drought, soil nutrition, drought, soil compaction, or root compaction, or root diseasedisease

Root Disease Root Disease SymptomsSymptoms

• WiltingWilting• StuntingStunting• Leaf yellowing and Leaf yellowing and

dropdrop• Softening and Softening and

discoloration of discoloration of roots and stems, roots and stems,

• Branch dieback Branch dieback • Plant deathPlant death

Pythium root rotPythium root rot• Most plants you

buy already have some root rot

• Prevent root rot: – Plant high– Improve soil

drainage– Redirect water– Do not over-water– Do not over-

fertilize– Right plant/right

placeArmillaria

• Root rot diseases in the landscape are often Root rot diseases in the landscape are often associated with improper planting, irrigation, associated with improper planting, irrigation, or site preparationor site preparation

Juniper DiebackJunipers like dry, sunny conditions, and do not like poorly

drained sites

Oak leaf blisterOak leaf blister

Spot Spot anthracnoseanthracnose

• Phyllosticta leaf spot Phyllosticta leaf spot

• Discula leaf spot (birch)Discula leaf spot (birch)

Most fungal leaf spots are mostly an Most fungal leaf spots are mostly an aesthetic problemaesthetic problem

Cercospora leaf spot on Hydrangea

Black spot of rose

Leaf Symptoms

Management of Black Spot

• Sanitation (destroying leaves, cutting back diseased canes)

• Mulching each year• Resistant cultivars!!• Increase air flow, plant in full sun• Keep leaves dry when irrigating• Fungicide Sprays (protectants every 7

to 12 days or systemics)

Seiridium canker on Leyland CyprusSeiridium canker on Leyland Cyprus

• Very commonVery common• Drought-stressed and wounded treesDrought-stressed and wounded trees• Irrigate trees during periods of droughtIrrigate trees during periods of drought

Mycosphaerella leaf spot on iris

Entomosporium leaf spot on red-tip (Photinia)

Prolonged leaf wetness, high humidity and poor Prolonged leaf wetness, high humidity and poor air circulation increases foliar diseasesair circulation increases foliar diseases

Defoliation on Photinia due to Entomosporium leaf Defoliation on Photinia due to Entomosporium leaf spot disease, overcrowding, and poor air circulationspot disease, overcrowding, and poor air circulation

Entomosporium on Indian HawthorneResistant Cultivars Available

Powdery Mildew

• Reduces yield and weakens plants• Surface mycelium – easy to diagnose• Some survive as spores in debris, others blow

in each year (cucurbit powdery mildew)• Management:

– Damage is minimal in most plants– Some cultivars with resistance– Increase air circulation– Sulfur - oldest fungicide, not very efficacious– Sodium and Potassium Bicarbonate– Ultrafine oils interfere with infection– Biologicals

Powdery mildewPowdery mildew

Rust diseases• Very complex lifecycles

– Autoecious: one host (geranium rust)– Heteroecious: two hosts (daylily rust,

cedar-apple rust, fusiform rust)

• Spores are spread by wind and water-splashing

• Fungicides usually not needed for landscape plantings.

• Resistant cultivars are better options in crop plants.

Daylily rust

• Yellow spots on upper side of leaf

• Orange spores directly beneath on leaf underside

• Destroy infected leaves

• Resistant cultivars

• Fungicides

Cedar-apple rust

• Orange telial horns disperse spores to apples, crabapples in spring

• Leaf spots and fruit distortion occur on apple/crabapple

• Spores then spread back to cedar in summer

Cedar apple rust on

crabapple

Cedar Quince Rust

Fire blight of apple, pear, crabapple, pyracantha, cotoneaster, photinia

• Occurs in warmer, wet springs• Secondary summer infections can occur

during wet years• Prune affected branches to reduce spread

• ““We” cause the majority We” cause the majority of tree decline problemsof tree decline problems

• Construction damage, Construction damage, mechanical injury, mechanical injury, improper planting, improper planting, girdling roots, and girdling roots, and compacted soils all compacted soils all stress treesstress trees

• Natural events also Natural events also contribute, i.e. drought, contribute, i.e. drought, lightning, fire, etc.lightning, fire, etc.

• Disease and insects Disease and insects

What’s wrong with my tree?

Red Maple with Gradual

DiebackCan you spot the cause of this decline?

Bacterial Scorch of Sycamore – many tree species are

susceptible, no cure

Phytoplasma Disease – Aster Yellows

• Cause stunting and color deformities

• Systemic bacteria in phloem• Interfere with growth and

development

Viruses – no cure, inspect planting material

Ring spot symptom – always a virus disease

Herbicide Damage

• Root-knot nematodes are common• Plants are stunted, roots galled• Healthy plants can resist infections• Some plant species more resistant

• Root-knot nematodes are common• Plants are stunted, roots galled• Healthy plants can resist infections• Some plant species more resistant

When a disease problem is suspected, obtain background information:

• Pattern in landscape• Number of plants affected• Part of plant affected• Irrigation time/frequency/areas• Recent weather conditions• Chemicals used on or near the site, rates• Fertilizer applied (rate, form of application)• Insects present