Updated: Nov 18, 2020
Yellow leaves on dahlias are USUALLY down to one of three things. Read on to get an an idea of what can be the cause.
If you notice your dahlias turning yellow early in their growth cycle, it’s likely down to bugs. They are the number one cause of plant damage and issues and thankfully also the easiest to solve!
Sap sucking insects like aphids, thrips and spider mites love this temperature change in spring and they run absolutely rampant on our dahlias. Aphids will cause yellowing on your leaves, and make it look like it’s wilting. They will suck the life out of it very fast and spread like wildfire and they leave your plant susceptible to mosaic disease.
Adult thrips are tiny, slender insects, 1/25 inch long, variously colored pale yellow, black or brown, with four long, narrow wings. Their larvae are usually wingless. Thrips infest flowers and plants by rasping the surface and feeding on the exuding juice. Necrotic spot virus is caused by the feeding of thrips and shows by small spots that turn into holes forming on the leaves. Silver coloured leaves are also an indication of thrips.
spider mites are the same and will also cause silvery discoloured leaves, leaving tiny thin webs all over the leaves, and need to be sorted out before they destroy them. If you want to deal with them asap- use your fingers and rub them off! Repeat daily until sorted. A spray with soapy water or diluted neem oil will help as well ( or any pesticide designed to treat them ) . long term environmentally safe measures include finding praying mantis , lady bugs and small spiders and transferring them to the affected plants ( make sure it’s not the harlequin lady bug which is not beneficial to the garden ).
Stem rot and tuber rot occurs when dahlias are growing in heavy, poorly drained, wet soil. It often occurs in potted dahlias or after a long period of rainy weather, and is the second most common reason after bugs for yellowing leaves, so if you’ve not spotted any bugs then definitely pays to check your tubers are ok.
Problems with dahlias also include verticillium wilt which affects the stems and turns them black and soft and mushy and the leaves turn brown, wilted and withered.
They love full sun locations in soil which free draining, loamy and is slightly acidic. A way to help this is wait until they’re dormant in winter and top dress it with garden lime and lots of animal manure to let the rain soak it in.
Lack of magnesium or iron will also cause yellowing dahlia plants.magnesium issues can be easily fixed by watering with Epsom salts - a teaspoon to a 9 litre can is more than enough.
Dahlia smut is caused by the fungus Entyloma dahliae. Symptoms appear as pale spots usually beginning on the lower leaves during cool, wet conditions due to rainfall or frequent overhead irrigation. Light to dark brown spots can be seen in the center of these white spots. As the spots mature, spores appear on the surface of each spot. This fungus requires very moist conditions to infect and spread. Fungicides effective against rust should be effective to prevent further infections. Reduce the density of the plant canopy, improve air circulation and light penetration, and reduce leaf wetness periods. Remove and destroy affected plants and all symptomatic leaves, and treat with a fungicide. Dahlia smut can over-winter in soil attached to stored tubers. Treat tubers overwintered indoors with a fungicidal dust before planting.
Finally, if none of these are the culprit, then it is possible it’s mosaic disease.
Mosaic virus is spread by Aphids, hence needing to get on top of aphids as soon as you can.
Some of the signs are:
It will be twisted, dwarfed and stunted growth, patchy spotty leaves
Chlorophyll loss resulting in light-colored, pale green to yellow streaks along the branch and veining.
Once a dahlia has been infected with the virus, it enters the plant’s cells and begins multiplying. This makes treating dahlia mosaic infected plants impossible. To prevent spread of the virus, it’s safest to remove dahlias with mosaic virus. Luckily, Dahlias with the mosaic virus can’t directly infect other dahlia plants. The virus is only spread by sap from an infected dahlia to a wound or opening in an uninfected one.