Pests
Symptoms Aboveground you often see nothing until it is bad, when the leaves may look rusty, wilted, or stunted. The damage is underground: small stiff white maggots tunnel just under the surface of the lower part of the root, leaving winding tunnels filled with rusty-brown mush. Affected carrots are scarred, taste off, and tend to rot in storage.There are usually no aboveground symptoms until damage is advanced, when foliage may look rusty, wilted, or stunted. The real signs are in the root: the larvae, small stiff white maggots, mine surface tunnels through the lower portion of the taproot, and the tunnels fill with a rusty-brown mush. Damaged roots are scarred, off-flavored, and prone to secondary rot in storage. Cause The maggots (young larvae) of the carrot rust fly, a small fly. The female is drawn to the smell of carrot leaves and lays her eggs near the base of the plant, and the maggots that hatch burrow into the roots. It also goes after parsnip, celery, and parsley. It is mainly a problem in cool, damp, coastal areas and much less common where summers are hot.Larvae of the carrot rust fly (Psila rosae), a small fly whose females are drawn to the scent of carrot foliage and lay eggs near the crown; the hatching maggots burrow into the roots. It also attacks parsnip, celery, parsley, and other carrot-family plants. The fly is a weak flier with localized populations and is most damaging in cool, humid, maritime regions, far less so in hot-summer areas. Organic treatment You cannot fix roots that are already tunneled, just cut away the damaged parts before eating. To limit more damage, harvest on time instead of leaving carrots in the ground, throw away (do not compost) badly infested roots, and clear out any wild or leftover carrots nearby that keep the flies going.There is no home rescue for roots already mined; trim damaged portions before use. Limit further damage by harvesting promptly rather than leaving roots in the ground, removing and not composting infested roots, and clearing nearby wild or volunteer carrots that sustain the population. Prevention The best home defense is a row cover, a light fabric or fine mesh laid over the bed right after sowing and tucked in at the edges, because the low-flying female cannot get to the plants to lay eggs. Also move carrots well away from last year's spot, time planting to dodge the main egg-laying periods (in cold areas sow in early summer; in mild areas, in late winter or in late summer), harvest a block at a time, and do not leave carrots in the ground over winter where this fly is active.The most reliable home defense is a floating row cover or fine mesh set over the bed at sowing and sealed at the edges, since the low-flying females cannot reach the crowns to lay. Rotate carrots well away from last year's bed, time sowings to miss the main egg-laying flights (in cold areas an early-summer sowing; in mild areas, in late winter or in late summer), harvest in blocks, and do not overwinter roots in the ground where the fly is active.
Symptoms The giveaway is little firm bumps or knots on the small side roots, called galls, which is different from the soft, fine hairy roots of aster yellows. The main root forks and twists into a deformed, often many-pronged carrot, because the tiny worms enter the root tip and disrupt it. Aboveground, plants in bad spots are stunted and yellowish and pull up easily. Northern root-knot nematode makes small knots; some other kinds make bigger, more obvious ones.The diagnostic sign is galling: firm, beadlike knots on the feeder and lateral roots, distinct from the fine, soft secondary roots of aster yellows. The taproot forks, twists, and grows malformed, often with several prongs, because the nematodes enter the root tip and disrupt it. Aboveground, plants in affected patches are stunted and yellowish and pull from the soil easily. Galls from northern root-knot nematode (Meloidogyne hapla) are small; other Meloidogyne species make larger, more conspicuous galls. Cause Microscopic worms in the soil called root-knot nematodes (in the genus Meloidogyne). They are most active and most damaging in warm, sandy soil and go quiet in cool soil, which is why carrots grown in cool weather or winter often come through fine. Carrots are one of the crops they hit hardest, and they live in the soil and feed on many different plants.Microscopic soil roundworms in the genus Meloidogyne (the root-knot nematodes), including the northern species M. hapla and warm-climate species such as M. incognita and M. javanica. They are most active and damaging in warm, sandy soils and largely inactive in cool soil, which is why cool-season and winter carrots often escape. Carrots are among the more susceptible garden crops, and the nematodes persist in soil and feed on many hosts. Organic treatment There is no home cure once the soil is infested, so pull up and throw away affected carrots instead of composting them. Plan around it next time: grow carrots in the cool season when the worms are inactive, rotate to other crops, and add organic matter to the soil, which helps the natural enemies that keep nematodes down.There is no home cure once a crop is infested; pull and dispose of affected carrots rather than composting them. Manage for future seasons: where infestation is known, grow carrots in the cool part of the year when nematodes are inactive, rotate to non-host or less-susceptible crops, and build soil organic matter, which supports the natural enemies that suppress nematodes. Prevention If your soil is warm and sandy and prone to nematodes, plant carrots so they grow in cool soil (below about 55 to 64°F), since the worms do not attack roots when the soil is cool. Keep carrots out of beds that have had root-knot problems, do not plant them right after other crops the worms love, add organic matter, and in bad cases use a raised bed filled with clean soil.In warm, sandy soils prone to nematodes, time carrot sowings so the roots develop in cool soil, below roughly 55 to 64°F depending on the species, since the nematodes do not invade roots in cool soil. Rotate away from beds with a history of root-knot, avoid following known host crops, build soil organic matter, and where pressure is severe consider raised beds with clean soil.
Diseases
Symptoms You will see spots on the leaves. Alternaria blight makes dark brown to black ragged spots with a yellow edge, starting on the older, lower leaves. Cercospora blight makes tan or gray round spots and can show up on younger leaves too. As the spots spread and join together, the leaves look burned and can die back. The two are hard to tell apart, and you treat them the same way. The carrots underground are not infected directly, but if the leaves get badly damaged the roots stay small.Dark brown to black, irregularly shaped spots with yellow halos appear first on the older, lower leaflet margins and tips; this is Alternaria dauci. Tan to gray circular spots with darker margins, showing on younger leaves and earlier in the season, point to the closely related Cercospora carotae. As the spots multiply they run together and the foliage takes on a scorched, blighted look, and large lesions can girdle and kill the petioles. The two fungi are difficult to tell apart and are managed identically. The roots are not infected directly but size up poorly when the canopy is badly damaged. A third, bacterial blight (Xanthomonas campestris pv. carotae), looks similar, with water-soaked, varnished lesions that track down the veins. Cause Fungi that travel on the seed and survive on old carrot debris and on wild carrots nearby. They spread in splashing water and need wet leaves to take hold, so long spells of wet foliage from rain, dew, or overhead watering bring them on, especially in warm, humid weather.Two fungi, Alternaria dauci and Cercospora carotae, that are seedborne and also survive on infected crop debris and on wild carrot. They spread in splashing water and need leaf wetness to infect, so prolonged wet foliage from rain, dew, or overhead watering drives the disease, especially in warm, humid weather. Organic treatment You cannot cure leaves that are already infected, only slow it down. Pick off and throw away (do not compost) the worst leaves, thin crowded plants so air can move through, and water at the soil in the morning so the leaves dry fast. In a very wet year a copper fungicide labeled for vegetables can help if you start early, but keeping the leaves dry and the plants uncrowded does most of the work.There is no cure for tissue already infected; management slows the spread. Remove and destroy badly infected leaves, thin to proper spacing to improve airflow, and water at the base in the morning so foliage dries quickly. In a wet season, a copper-based fungicide labeled for vegetables can suppress spread if started early, but cultural control is the mainstay for a home crop. Prevention Use fresh, clean seed, and do not grow carrots (or relatives like parsley and dill) in the same spot two years in a row. Clear away old carrot tops and any wild carrots nearby, since the fungi live on them. Water at the base instead of overhead, give plants room so the leaves dry, and pick a blight-tolerant variety if it keeps coming back.Start with clean or treated seed, rotate carrots and other carrot-family crops out of the bed for at least two years, and clear crop debris and volunteer or wild carrots that harbor the fungi. Avoid overhead watering, space and thin for airflow, and choose tolerant varieties where blight recurs.
Symptoms An infected carrot grows strangely: the new leaves turn yellow, then reddish-purple, twist, and bunch into a weak clump, and the root underneath comes out pale and tapered with a thick fuzz of fine hairy roots. The carrot tastes bitter and is not good to eat. You will usually see just a scattered plant here and there, not a spreading patch, and there is no way to cure a plant once it has it.Infected carrots push up yellowed, then reddish-purple, twisted new foliage that clusters into a weak, bunchy top, while the taproot turns pale and tapered and grows a dense mat of fine hairy secondary roots. The root tastes bitter and is inedible. Affected plants usually appear scattered singly through a planting rather than spreading plant to plant, and there is no cure once a plant is infected. Cause It is caused by a phytoplasma (a bacterium-like organism) that lives inside the plant. A small insect called the aster leafhopper carries the phytoplasma from infected weeds or plants and passes it on when it feeds on your carrots. Weeds like wild carrot, dandelion, and thistle can hold it and pass it along.A phytoplasma, a bacterium-like organism that lives in the plant's food-conducting tissue, spread by the aster leafhopper. The insect picks the pathogen up from infected weeds or crops, often arriving on migrating leafhoppers, and injects it while feeding. Many weeds, including wild carrot, dandelion, and thistle, serve as reservoirs. Organic treatment There is no way to cure a plant that already has it. As soon as you spot one, pull it up and throw it away, not in the compost, so leafhoppers cannot pick it up from that plant and carry it to your other carrots.No treatment cures an infected plant. Pull and destroy symptomatic plants promptly so they cannot serve as a source for leafhoppers to spread it further, and do not compost them. Prevention Keep weeds down in and around the garden, especially wild carrot and dandelion, since they shelter both the germ and the insect that spreads it. A row cover, a light fabric laid over the plants, keeps the leafhopper off, and pulling any sick plant right away keeps it from spreading. How common this is depends on your area and the year.Control broadleaf weeds in and around the garden that host the phytoplasma and the leafhopper, especially wild carrot and dandelion. Row covers exclude the leafhopper from a planting, and prompt removal of any infected plant limits a local reservoir. Risk is highest where migrating aster leafhoppers are abundant, which varies by region and year.
Symptoms Small sunken oval pits, usually only a few millimeters long, on the surface of the carrot root, sometimes turning dark or cracking. The leaves look fine, so you usually notice it only when you pull and wash the carrots. It does not kill the plant; it just blemishes the roots. It mostly turns up in larger in-ground rows on wet, heavy, poorly drained soil, so a well-drained raised bed is low risk.Sunken, elliptical lesions a few millimeters long on the taproot surface, sometimes darkening or cracking. There are no aboveground symptoms, so it is found at harvest. It is a quality and storage problem rather than a plant-killer, and it tends to show up in larger in-ground plantings on wet, heavy, poorly drained soil rather than in a well-drained raised bed. Cause Tiny soil organisms called Pythium, which are fungus-like water molds (mainly Pythium violae and Pythium sulcatum), that infect the growing root. They thrive where soil stays too wet, drains poorly, or is too acidic, and carrots left in the ground a long time are more likely to get it. They survive in soil for years and attack many plants, so simply rotating crops does not get rid of them.Soilborne Pythium species, chiefly Pythium violae and Pythium sulcatum, that infect the developing root. The disease is favored by prolonged wet soil, acidic conditions, and poor drainage, and older roots left in the ground longer are more susceptible. The pathogens persist in soil as resting spores and have a wide host range, so rotation alone does not eliminate them. Organic treatment There is no spray that fixes roots once they are infected. The fix is in how you grow them: improve drainage, do not overwater, and harvest on time instead of leaving mature carrots sitting in wet soil.There is no rescue treatment once roots are infected. Management is preventive and cultural: improve drainage, avoid overwatering, and harvest promptly rather than holding mature roots in wet ground. Prevention Plant in loose, well-drained soil, and use a raised bed if your ground stays soggy. Do not overwater, keep the soil pH in the right range (around 6.0 to 6.8, not too acidic), and harvest carrots when they are ready instead of leaving them in wet ground, since the longer they sit the worse it gets.Grow carrots in loose, well-drained soil, raise beds where drainage is poor, avoid overwatering, and keep soil pH in the recommended slightly acidic to neutral range rather than letting it run low. Harvest at maturity, since the longer roots stay in infested wet soil the worse it gets, and choose tolerant varieties where there is a history of the disease.
Symptoms Seeds that never come up, or tiny seedlings that flop over and die soon after sprouting, usually with a dark, pinched, water-soaked spot on the stem right at the soil line. You often see it as bare gaps in the row. Because carrot seed is slow and the bed is kept damp for 2 to 3 weeks, conditions favor it; the same organism can also leave young roots stubby and forked if it kills the root tip early.Seeds fail to emerge, or young seedlings topple and die soon after emergence, typically with a dark, water-soaked, shriveled spot at the stem base right at the soil line. Losses often appear as bare patches in the row. Because carrot germination is slow, often 2 to 3 weeks, the seedbed is held moist for a long time, which is exactly the condition the pathogens exploit; the same Pythium can also leave young roots stubby and branched if it kills the root tip early. Cause Soil organisms (mostly Pythium, a fungus-like water mold, plus a few others) that attack sprouting seeds and baby seedlings. They thrive in cool, wet, poorly drained soil and when you overwater, and they move in soil water. Old or weak seed and a hard, soggy seedbed make it worse.Soilborne pathogens, chiefly Pythium species along with Rhizoctonia and others, that attack germinating seed and young seedlings. They are favored by cool, wet, poorly drained soil and by overwatering, and they spread in soil water. Old or weak seed and a crusted, slow-draining seedbed increase losses. Organic treatment Seedlings that have collapsed will not recover. If it shows up, stop watering for a bit and let the soil surface dry out some to slow it down, and clear away the fallen seedlings. Mostly, though, this one is about prevention, because there is no good home cure once it starts.Affected seedlings cannot be saved. If damping-off appears, stop watering and let the surface dry somewhat to slow the pathogens, and clear out collapsed seedlings. The real lever is prevention, since there is no effective home cure once it takes hold. Prevention Wait to sow until the soil is warm enough and not cold and soggy, and keep the seedbed evenly damp without letting it stay soaking wet. Use fresh seed, do not plant too deep, improve drainage or use a raised bed if your soil is heavy, and resist the urge to overwater during the long wait for the seeds to come up.Sow into warm-enough, well-drained soil rather than cold, wet ground, and keep the seedbed evenly moist without keeping it saturated, maintaining moisture at seed depth rather than soaking the surface. Use fresh seed, avoid sowing too deep, improve drainage or use raised beds on heavy soil, and do not overwater during the long germination window.
