1 of 52

Common Pests & Disease in Dahlias

Marjorie Meyer

June 1, 2024

Mike Clemente’s 2023 Photo of Crazy Legs

2 of 52

Slugs & Snails - like it cool and moist

3 of 52

Organic Treatment for slugs and snails

Set out traps like flat boards, shingles or damp newspapers then kill the hiding slugs
Drown by setting out water, yeast or beer traps
Use copper as a barrier around plants
Invite their enemies in (ground beetles, rove beetles, fireflies), toads, snakes, turtles, shrews, ducks, starlings and other birds
Put egg shells, spiny gumball seeds or diatomaceous earth under and around your plants

4 of 52

Chemical Treatments for slugs and snails

Important: Metaldehyde is available for use. It is very toxic to dogs and other animals.

Iron phosphate -a stomach poison that damages the slug’s digestive system. (Sluggo)

Less toxic bait for slugs; it does not pose problems when used around children and pets.

Slugs stop feeding several days after eating iron phosphate and eventually die (up to seven days).

Iron phosphate is effective for two weeks, even if it becomes wet.

Ferric sodium EDTA

A less toxic product similar to iron phosphate.

It works more quickly, killing slugs in just a few days

5 of 52

Caterpillars

Leave frass - can hand pick it off.

Leave irregularly shaped holes in leaves

Nice words for frass:

Excrement

Castings

6 of 52

Organic Methods of Control

The best strategy for controlling caterpillars is to handpick them, then crush them or drop them in a pail of soapy water.
Make sure you also remove rolled or folded leaves that shelter the caterpillars.
Dust or spray all parts of the leaves on which they are feeding (especially the undersides) with BT (Bacillus thuringiensis) or spinosad. Apply at 3-5 day intervals, reapplying after rain, until caterpillars cease to be a problem.
Spinosad is a more broad-spectrum microbial insecticide. The wet spray of Spinosad, for example, is toxic to bees and other beneficial insects. They are minimally affected once Spinosad residue is dry.

Chemical Methods of Control

carbaryl (Sevin SL)

Malathion (Malathion)

pyrethrins (Pyganic)

pyrethrins +

azadirachtin (Azera)

spinosad (Entrust)

Bacillus thuringiensis

(DiPel ES)

methoxyfenozide

(Intrepid)

Burkholderia spp

(Venerate CG)

Chromobacterium

subtsugae (Grandevo)

azadirachtin

(Aza-Direct)

7 of 52

Use up-to-date references

8 of 52

Pests - Aphids

Three types attack dahlias

Could be red, yellow, green, black or brown in color.

9 of 52

Yellowing leaves, stunted growth and/or black sticky substance.

Have a tube-like structure projecting from the hind end of the body.

Produce honeydew that encourages a black sooty mold.

10 of 52

Organic

Neem (azadirachtin) is a plant based pesticide that discourages aphid feeding. Insects are not killed quickly, but it makes them stop feeding and they slowly die.

Insecticidal soap, horticultural oil and pyrethrins can be effective at controlling aphids.

Blast off with water spray on the top and bottom of the leaves.

Chemical

Foliar applications of malathion or diazinon should adequately control aphids.

Applying the granular systemic insecticide, disulfoton (Di-Syston two percent granules), to the soil provides extended control (six to eight weeks) without the necessity of frequent applications.

Di-Syston also is effective against other sucking insects.

Aphid Treatments

11 of 52

Japanese Beetles

12 of 52

Proper practices, e.g., irrigation, fertilization, mulching, will help keep plants healthy, so they can tolerate minor infestations.

Adult beetles live and feed around 2 months.

Don’t fall into the the Japanese Beetle trapping trap!

Traps are scented with beetle pheromones (scent to mate), flowers or both. These smells travel through the air. The scent is picked up on beetle antenna. Beetles end up both inside and outside the trap. As large numbers of beetles congregate, they put off more pheromones that attract more beetles, whether or not they are in the trap

They are good fliers! In a single flight easily going a couple of miles! Often found 10 to 15 miles away from their grub birth place.

13 of 52

Nematodes—microscopic parasitic roundworms—actively seek out grubs in the soil.

Heterorhabditis bacteriophora latter is commercially available.

Bacillus thuringiensis (Bt)—Bt is a naturally occurring soil bacterium typically used as a microbial insecticide. The Bt strain registered for the Japanese beetle is for use on the grub stage only. Bt is a stomach poison and must be ingested to be effective. Apply it to the soil as you would insecticides.

Milky Spore—Milky spore is the common name for spores of the bacterium Bacillus popillae. Spores germinate in the grub’s gut, infect the gut cells, and enter the blood. Milky spore disease builds up in turf slowly (over 2 to 4 years) as grubs ingest the spores, become infected, and die, each releasing 1–2 billion spores back into the soil.

Parasites—Releasing natural enemies or parasites of an exotic insect aren’t yet commercially available, but are being used in the Northeast down to North Carolina.

Grub (the larval stage) Treatments

Chemicals for Larvae:

Imidacloprid (Merit ®)
Halofenozide
Trichlorfon

Trichlorfon

14 of 52

Make your habitat less suitable for Japanese Beetles:

Use resistant plants or move clear from those susceptible to feeding.

ToResistent Trees

1. Red maple Acer rubrum

2. Boxwood Buxus spp.

3. Hickory Carya spp.

4. Redbud Cercis spp.

5. Tulip poplar Liriodendron tulipifera

ToResistent Plants

1. Ageratum Ageratum spp.

2. Columbine Aquilegia spp.

3. Dusty-miller Centaurea cineraria,

4. Begonia Begonia spp.

5. Lily-of-the-valley Convallaria majalis

ToSuspectable Trees

1. Japanese maple Acer palmatum

2. Norway maple Acer platanoides

3. Crape-myrtle Lagerstroemia indica

4. Apple, crabapple Malus spp.

5. Pin oak Quercus palustris

TooSuspectable Plants

1. Hollyhock Alcea rosea

2. Dahlia Dahlia spp.

3. Hibiscus Hibiscus moscheutos

4. Common mallow Malva rotundiflora

5. Evening-primrose Oenothera biennis

15 of 52

Organic

Handpicking beetles and dropping them into a can of soapy water. Do this in the morning, when beetles are less active.

Chemical

Chemical sprays are effective in reducing feeding. Recommended insecticides include Cyfluthrin, Bifenthrin, and Cyhalothrin. These products will need to be reapplied about every couple of weeks during the feeding period,

When using pesticides, read and follow all label directions including the use of personal protective equipment.

Neem and Pyola will give some control of the adult beetles, but only for about 3-7 days.

Adult Stage Treatments

16 of 52

Need the perfect bloom when the beetles are at war with you?

Organza wedding favor bags

Pollen isolation bags of many sizes can be found online.

17 of 52

Spider Mites!

18 of 52

Barely visible to the naked eye

Their name comes from the fine silken webs they produce.

19 of 52

We often notice when browning shows at the top of the leaves.

20 of 52

If damage is extensive, remove the plant to the landfill.

If limited, put damaged leaves and healthy leaves surrounding them in the trash bin.

Using a water hose, spray blast other mites away, focusing on the underside of the leaves.

The action is mainly on the underside of the leaves.

Feeding empties the cell contents of chlorophyll. Without it, the empy cells appear whitish or bronze.

21 of 52

20 eggs per day are laid

Males are produced from the unfertilized eggs.

Females from the fertile eggs.

Larvae will hatch from eggs after 3 to 15 days

Mites mature into adults in as few as 5 days

The large transparent ovals are eggs laid by the surrounding females.

22 of 52

Oils smother soft bodied insects and arachnids. Insects must be present for oils to work!

Apply horticultural oil safely only when temperatures are less than 85 degrees.

23 of 52

Prevent where possible

Keep your plants healthy:

Well watered

Adequate spacing

Fertilized appropriately

Get pests and diseases correctly diagnosed.

Use the right treatment for the problem.

Contact your local Extension Service for help.

My experiment - plant sweet alyssum near plants. My mites preferred the sweet smelling little flowers over non-scented dahlias so far!

24 of 52

Management-chemical control: HOME USE

azadirachtin
bifenthrin
carbaryl
horticultural oils
insecticidal soap
malathion
plant essential oils (clove, garlic, peppermint, rosemary, thyme) and cottonseed and garlic oils
Pyrethrins
sulfur

25 of 52

Spider Mites can be difficult to control with chemicals.

They are resistant to most pyrethroids and in some cases certain chemicals can increase the egg laying rates. Rotating miticides among mode of action group numbers will help prevent resistance. It is best to keep in mind the life cycle of the spider mite that a chemical targets because some chemicals affect the eggs while others focus on mobile growth stages.

Some insecticides ‘Flare’ Spider Mites: One of the most important points to keep in mind about spider mites is that some insecticides will actually ‘flare’ spider mites (cause a population explosion). This can occur in two ways. Some insecticides are more toxic to predatory mites and other insects that feed on spider mites than they are to the spider mites. Kill the predators and spider mite numbers increase rapidly. The other reason is a phenomenon known as hormoligosis, the insecticide actually causes the spider mites to reproduce faster.

Many of the insecticides commonly used to control insect pests tend to flare spider mites. Foliar sprays of acephate or carbaryl are especially likely to flare mites. Sometimes it seems as if there must have been spider mite eggs in the insecticide bottle. Most of the pyrethroid insecticides (permethrin, cyfluthrin, lambda-cyhalothrin, etc) also flare mites.

26 of 52

Earwhigs

Main source of food is other insects

Like to hide in dark, tight fitting areas
Active and feed at night

27 of 52

Organic

Trap/drown using vegetable oil in a small container buried in the soil. Fill oil only to about 1” from the top..

Can be trapped in rolled-up moistened newspaper or rolls of cardboard.

Chemical

Foliar applications of bifentrin, permethrin, lambdacyhalothrin.

Applying baits, containing carbaryl

Di-Syston also is effective against other sucking insects.

Earwhig Treatments

28 of 52

Thrips

29 of 52

Organic

Hose plants with water in the morning.

Beneficial bugs, mites and nematodes are available

Remove damaged leaves, buds and flowers to control population

Neem oil and insecticidal soap may be helpful

Chemical

Avoid pyrethrins, as thrips have become resistant.

Applying carbayl (Sevin) may result in spider mite flare-ups.

Chemical treatments:

Acetamiprid (Assaill)

Spinetoram, a new type of synthesized type of Spinosad (Radiant)

Thrips Treatments

30 of 52

Predatory Insects May Work Well in Greenhouses

They can fly away, die because of conditions, or not get along with neighbors!

In urban areas they often become victims of chemical weapons.

31 of 52

Striped cucumber beetle Acalymma vittatum

Spotted cucumber beetle Diabrotica undecimpunctata howardi

32 of 52

Adults lay eggs at the base of plants, which hatch in 7 to 10 days,

Larvae appear to have two heads. They feed in roots for about 3 weeks before pupating in the soil.

Cucumber beetles overwinter as fertilized females. They become active in early spring. There are two generations a year.

Adults emerge 2 weeks later and feed on pollen, foliage, flowers, and pods.

Beside feeding on cucumbers and other melon crops the adults love to feed on the flowers of dahlias, zinnias, sunflowers and occasionally marigolds

33 of 52

Striped cucumber beetles leave their overwinter site and Move into gardens when the air temperature is greater than 54°F.
Adults feed on cucurbit plants, and then the females and males mate. females lay 200 to 1,200 eggs in the soil at the base of cucurbit plants.
Larvae emerge (eclose) from eggs and feed on plant roots for two to three weeks and then pupate

in the soil.

Striped cucumber beetle adults emerge in late

summer to start a second generation.

Striped cucumber beetle, Acalymma vittatum

Spotted cucumber beetle, Diabrotica undecimpunctata howardi

Spotted cucumber beetle adults do

not overwinter in northern areas but instead migrate from southern states each year.

Adult females lay eggs in the soil among corn and

grasses, so larvae do not feed on the roots of cucurbit plants.

There can be two or more generations during the

growing season.

34 of 52

Organic Control Methods:

Insecticidal sprays may be needed if populations of cucumber beetles are high.
Treat early in the season before overwintering beetles have a chance to lay eggs.
Pesticides registered for use include:

carbaryl (Sevin)
endosulfan (Thiodan)
Permethrin and pyrethrins.

Conventional or broad spectrum pesticides

Remove cucumber beetle adults by hand.

Sanitation practices such as the removal of crop debris or residues will help reduce populations of overwintering adults.

Place yellow sticky cards among cucurbit crops to capture cucumber beetle adults.

35 of 52

Grasshoppers

36 of 52

Organic Treatment

Hand pick into soapy water.

Grasshoppers are among the most difficult insect pests to manage in the garden.

Screened boxes, floating row covers, and other protective covers provide some protection if the number of pests isn't high

Poultry, including chickens and guinea hens, are excellent predators but can they also cause their own damage the garden.

37 of 52

Conventional or broad-spectrum pesticides

Table I. Recommended active ingredients for products designed for grasshopper control in and around the yard and garden. Always check label for application instructions, rates and safety precautions.

carbaryl (available in several liquid or dry formulations)
carbaryl (available as a bait formulation on wheat bran)
acephate (available in several formulations)
bifenthrin (available in several formulations)
cyfluthrin (available in several formulations)
esfenvalerate (available in several formulations)
malathion (available in several formulations)
permethrin (available in several formulations)
lambda cyhalothrin (available in several formulations)
deltamethrin (available in several formulations)

Once grasshoppers have invaded the garden, insecticides won't be very effective and must be reapplied every few days as long as the invasion continues. Nearby areas would also need to be treated, especially in urban areas.

spotted cucumber beetle (Diabrotica undecimpunctata howardi)

38 of 52

Spray the frass off!

39 of 52

Diseases of Dahlias - Fungus

Powdery Mildew

common in warm, humid climates
shows up during the second half of the growing season
damage is largely cosmetic

Botrytis

gray mold
buds will become black

Verticillium Wilt - No cure or treatment

leaf margins may turn brown or appear scorched.
Stems may turn yellow, display brown or black streaks

40 of 52

Prevention is the cure

Space plants for good air circulation.

Remove and destroy severely infected plants or plant parts.

Treatment with fungicide must be applied as soon as symptoms are noticed.

Product labels will provide information on how often to spray and air temperature limits.

Be sure to cover both the upper and lower surfaces of the leaves.

41 of 52

Fungicides for Powdery Mildew Control

42 of 52

Crown Gall found in soil bacteria caused by the soil bacteria Agrobacterium tumefaciens.

Also found in 600 other plants, like peach, pear, rose, anemone, aster, geranium, heuchera, sedum, campanula, euonymus, willow.

Infection with A. tumefaciens causes

tissues to swell into tumors, or galls, on stems or roots. These galls do not transform into buds or stems.

Yarrow

Dahlia tuber

Euonymus

Dahlia tuber

Rose

43 of 52

Leafy Gall - Commonly Found

Rhodococcus fascians

A leafy gall is a mass of buds or short shoots tightly packed together and fused at the base. These may appear beneath the soil or near the soil line at the base of the

stem

Rhodococcus fascians

The Latin fascia means, to fuse

refers to a joining of tissues.

44 of 52

Bottom line on galls - bacteria is in the soil at least 2 years and spreads through wounds.

When in doubt throw it out!

There is not a treatment for either crown or leafy gall

We need to limit the spread of the bacterial disease

Start with clean stock

Sanitary practices to clean tools, plant rotation

They’re not just dahlia diseases:

Both are known to infect herbaceous and woody

plants. Both bacteria have a wide host range

(over 60 species for R. fascians, and hundreds

for A. tumefaciens).

45 of 52

Viral Diseases on Dahlias

The following are some of the virus problems on dahlias:

Tomato spotted wilt virus
Impatiens necrotic spot virus, which have a thrips vector,
Dahlia mosaic virus (DMV) and Dahlia common virus (DCVM) , which have an aphid vector,
Tomato-ringspot-related, which most likely has a nematode vector.

Images for each can be seen on this presentation .

46 of 52

Dahlia Common Mosaic Virus

DMV or DCMV?? DNA sequencing shows differences , but there might be an error.

Dahlia mosaic produces distinct chlorosis along veins and may shorten the internodes.

May have a stunted or bushy appearance

47 of 52

Dahlia Mosaic Virus

48 of 52

Certified virus-free stock with virologist Hanu Pappu, Ph.D.

Washington State University has teamed up with the American Dahlia Society to stop the spread of the Dahlia Mosaic Virus (DMV).

Dahlias are propagated from tubers and cuttings. If a mother plant is infected, its child plants will be infected, too, with viruses accumulating in dahlia crops over time. Pappu determined seeds also transmit DMV.

“By using tubers and cuttings, we’re unintentionally spreading viruses to the next generation,” said Pappu.

Dr. Pappu and WSU have rapid, sensitive viral detection methods at the Clean Dahlia Center. This will greatly benefit commercial growers and us to get tubers free from DMV.

49 of 52

Stop DMV Spreading

Buy certified virus-free seed and tubers to reduce virus incidence. Do not share tubers from plants that have been infected, as this will lead to more infected plant material.

Destroy symptomatic plants to prevent virus spread by aphids to healthy plants. Aphid control with insecticides is generally ineffective because aphids are mobile and infect many plants before succumbing to the treatment. Besides dahlias, various weeds, vegetables and other garden plants can be infected with the virus. Inspect and remove any nearby plants with similar symptoms.

When cutting flowers, sanitize tools with 10% bleach solution ,a dip in a 10% bleach

solution or a quick dip in a solution of 5 tsp of Dawn dishwashing soap in

4 cups of water or Virkon S which is a commercially available product that is commonly used in veterinarian clinic applications to kill virus.

50 of 52

References used

https://extension.colostate.edu/topic-areas/insects/spider-mites-5-507/

https://hort.extension.wisc.edu/articles/twospotted-spider-mite-tetranychus-urticae/

https://www.dahlia.org/wp-content/uploads/2023/05/Coping-with-Spider-MitesSpider-5-23.pdf

https://www.dahlia.org/wp-content/uploads/2021/03/Fertilizing-bug-disease.pdf

https://www.dahlia.org/wp-content/uploads/2023/05/Mite-Excerpt.pdf

https://www.dahlia.org/wp-content/uploads/2023/05/Broad-Mites.pdf

http://pnwhandbooks.stage.extension.oregonstate.edu/insect/hort

https://bearworks.missouristate.edu/cgi/viewcontent.cgi?article=1016&context=books

https://extension.umn.edu/yard-and-garden-insects/slugs#natural-enemies-can-help-control-slugs-298064

https://www.johnson.k-state.edu/lawn-garden/agent-articles/insects/japanese-beetles-in-kansas-city.html

https://bookstore.ksre.ksu.edu/pubs/MF3488.pdf

https://www.aphis.usda.gov/plant_health/plant_pest_info/jb/downloads/JBhandbook.pdf

https://www.arborday.org/trees/health/pests/japanese-beetle.cfm

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1972&context=extensionhist

https://extension.umn.edu/yard-and-garden-insects/cucumber-beetles

51 of 52