Methods of Suppression
Control weeds to avoid breeding grounds, and wintering space.
Follow Crop rotation guidelines
For indoor plants and seedlings - avoid putting susceptible plants close to windows, vents or doors. Bugs don't knock before they enter, if they did you probably wouldn't hear it anyway.
Destroy & discard infected plant material
Dark spots with yellowing borders develop on leaves and stems.
Alternaria leaf spot , a fungal disease is the likely cause. It overwinters in diseased debris in the soil and it may be spread on or in contaminated seed ,or by the wind. The disease usually starts on older leaf margins causing dark spots with yellow borders to develop.
Spots on the leaf stems will elongate and kill the entire leaf. Infection most commonly occurs during the cooler portions of the growing season when the leaf moisture does not dissipate as quickly.
Sulfur-based fungicides have proven effective against this disease.
Leaf surfaces are covered by a white powdery mass.
Powdery Mildew is the most likely culprit.
I've had success with Garlic GP Vegetable Fungicide on carrots in the past, and would recommend it.
Brown spots on leaves. Spots are small and round with a tan to black center and an indefinite yellow halo.
Cercospora leaf spot is caused by a fungus , it affects all foliage parts of the plant but not the roots. Infection usually first occurs on young leaves in the margins. Lesions are small and round with a tan to black center and an indefinite yellow halo.
Many spots on one leaf can cause withering and death. Infection of the leaf stalks results in pale centered, elliptical tan lesions. Disease development is favored under warm and humid weather. Remedy Fungicide is one of several fungicides that can be used against Cercospora leaf spot.
Roots are rotting , decayed and discolored when harvested
Root rot or cavity spot is a fungal disease that favors warmer soils. Remove any infected plants. Don’t let carrots sit in the garden as older carrot roots are more susceptible to root rot. Keep garden weeded and debris free. Rotate crops regularly.
Seeds rot without germinating or seedlings collapse with dark water-soaked stems as soon as they emerge.
Damping Off - a soil dwelling fungus is the probable cause. IT is favored by high humidity. Do not plant in cold, moist soil. Make sure soil is well drained. Avoid overcrowding plants.
Blackened spots on carrot roots that have already been harvested
Black Root rot of carrots is a post-harvest problem, although soil is the source of the pathogen.
This disease can be minimized by washing soil from them before storing and cooling them as quickly as possible to at least 45 degrees , and rinsing them in chlorinated water
Leaves are somewhat curled, deformed, and-or discolored. Plant growth is stunted.
Look for a sap-sucking insect. Aphids are the most common. Aphids are basically plant lice. They are Yellowish-pink to pale green, soft-bodied, oval~pear shaped insects and are commonly found on nearly all varieties of plants.
Fine Webbing on under sides of leaves. Discolored shoots - pale yellow to white.
Spider Mites are another sap sucking insect.
Spraying with water will rid the plant of them -temporarily- use insecticidal soap or rotenone for a more permanent control.
Roots are tunneled, tunnels contain a rusty sludge
Carrot rust fly is to blame, or more correctly its larvae.
The maggots of the Carrot Rust Fly damage plants by eating the small fibrous roots and by tunneling in larger roots. A rust-colored material develops in the tunnels, giving the insect its name.
Affected plants may become yellow, stunted, and die. Usually the plant tops continue to look healthy. Maggots often continue to feed in stored carrots. Disease organisms may enter the feeding tunnels and cause them to rot.
Neem has proven effective against this pest.
Sprinkling rock phosphate around the plant base will discourage egg laying - so they say.
Roots and stems are and chewed and gnawed into.
There are a number of insects that will behave in this manner, but the prime suspect is the Carrot Weevil
The Adult carrot Weevil is a dark-brown snout beetle about 6 mm long It over winters in plant debris in and about carrot fields that were infested the previous year.
They feed on foliage, chewing out notches, damage is usually not severe and the use of pesticides in backyard gardens for this purpose shouldn't be neccesary.
Predatory nematodes will parasitize the larvae . Spray any visible adult weevils with a pyrethrin/pyrethrum insecticide product as directed on the label.
Another possible culprit is the Celery WormAlso known as the carrot caterpillar. It is the larva of the black swallowtail butterfly. The adult butterfly is black with two rows of yellow spots on the outer border of the wings and light blue areas on the lower wings.
The most common host plants are carrots, celery, parsley, dill, and parsnip. This pest chews leaves and stems and destroys tops. Hand-picking the caterpillar is generally sufficient.
Small holes in leaves of seedlings and young plants early in the season,
Flea Beetles - These pests will eat small holes in the leaves of carrots and other root crops. They also carry plant diseases.Flea beetles are a difficult pests to manage causing headaches for the most avid gardeners, various varieties attack eggplant, seedlings of tomatoes, potatoes, peppers, radishes, and corn.
Cedarcide is an effective deterrent of flea Beetles and a large array of other insect pests.
Spreading diatomaceous earth around seedlings will also help.
Small black holes in Roots
Probably Wire Worms - the slender, yellowish-brown, hard-bodied larvae of click beetles.
There are many species of wireworm , various species attack Beans, Carrots, beet, celery, lettuce, Onion, sweet potato, turnip, and mustard . These worms reek their havoc by puncturing and tunneling stems, roots and tubers. Carrots are a particular favorite.
Plants bolt to flowering and go to seed.
Bolting is usually triggered by an extended cold spell - temperatures below 65°F - hot days and late frosts can also be contributing factor. Protecting young plants with cold caps, Cloches or floating row covers will help.
Losing some carrots to bolting is not uncommon in some regions, especially early in the season. Try planting succession sowings to ensure a constant harvest.
Strong flavor and woody carrots
Prolonged hot weather - dry weather - and/or lack of water
Carrots taste bitter and soapy
Carrots were probably harvested too early and never had the chance to develop their sugars.
As raw carrots they aren't worth much, but cooking or steaming them at low temperatures can bring out the sweetness.
Carrot tops are green.
When the roots tops are exposed to sunlight chlorophyll develops. Green Carrots are not edible. Cover the exposed roots with soil or mulch.
Carrots are pale orange.
Low temperatures , below 65°F will cause this. Avoid planting carrots too early in spring.
Elongated cracks in carrot roots.
Elongated cracks in carrot roots. Probable cause is inconsistent sporadic watering. Keep planting bed evenly moist. Mulching wouldn't hurt either - it helps to retain even soil moisture.
Another possible cause is boron deficiency. Test the soil and apply borax at a rate 1 tablespoon borax to 12 quarts of water to raise the boron level.
Cracked roots are still edible so long as no decay is present.See Plant Nutrient Deficiencies
Plants growth stunted . Leaves pale green. Older leaves yellow and develop red tints.
Frequently mistaken for a disease issue - it is actually a nitrogen deficiency
Nitrogen can be added to the soil by natural methods such as nitrogen fixing plants and green manure, but this will not help your plants in a timely fashion
They'll be dead before the nitrogen gets to the plant tissue.
Adding a high nitrogen fertilizer is the quickest way to ensure the nitrogen gets to your plants in a reasonable time.
See Understanding Fertilizer Labels