Pests
Symptoms You'll see ragged holes chewed in the leaves, green caterpillars on the plant, and dark droppings. The leaves are part of why you grow kohlrabi well, and the caterpillars can also chew the bulb.Ragged holes chewed in the leaves with green caterpillars and dark frass; heavy feeding sets young plants back and the larvae can score the bulb. Imported cabbageworms are velvety green, loopers move in a looping arch, and diamondback larvae are small and wriggle when disturbed. Cause These are caterpillars that hatch from eggs laid by white butterflies and small moths. They love cabbage-family plants like kohlrabi and chew the leaves.The larvae of the imported cabbageworm, cabbage looper, and diamondback moth feed on brassica foliage; the adults are white or brownish butterflies and moths that lay eggs on the leaves. Kohlrabi is a primary host. Organic treatment Spray Bt, which is safe and targets only caterpillars, or spinosad, and pick caterpillars and eggs off by hand from under the leaves. Spray again after rain.Spray Bt (Bacillus thuringiensis kurstaki) or spinosad, which target caterpillars and spare beneficials, and hand-pick larvae and eggs from leaf undersides. Repeat after rain, since Bt breaks down. Prevention Cover plants with a light fabric row cover after planting to keep the butterflies and moths from laying eggs, clear away old cabbage-family plants, and watch for the first white butterflies.Use floating row cover or fine mesh from planting to exclude the egg-laying adults, clean up brassica debris that shelters pupae, and scout for the first white butterflies as a spray cue.
Symptoms You'll see clusters of small gray-green insects with a dusty, waxy coating on the new growth and tucked into the folds of the leaves. Leaves may curl and there can be a sticky film.The gray-green, waxy-coated cabbage aphid forms dense colonies on growing tips and leaf undersides and in the leaf axils, curling and distorting the foliage and weakening young plants; heavy colonies foul the crop with honeydew. Cause Cabbage aphids are tiny insects that suck the plant's juices and breed fast. They love cabbage-family plants like kohlrabi and hide in the leaf folds.Brevicoryne brassicae, a brassica-specialist aphid, sucks sap and reproduces rapidly; winged forms move in from cruciferous weeds and other brassica crops. The turnip aphid can also occur. Organic treatment Knock colonies off with a strong spray of water and hit the undersides of leaves with insecticidal soap or neem oil, repeating every few days. Welcome ladybugs and lacewings.Dislodge colonies with a strong jet of water and spray insecticidal soap, neem, or horticultural oil with thorough coverage of leaf undersides; repeat every few days. Conserve natural enemies (parasitoid wasps, hoverflies, ladybugs). Prevention Check the growing tips and leaf folds often and act early. Go easy on nitrogen, since soft growth draws them, and clear away old cabbage-family plants after harvest.Scout growing tips and leaf axils weekly and catch infestations early. Avoid excess nitrogen, use row cover on young plants, and destroy crop debris that carries them over.
Symptoms Lots of tiny round holes in the leaves of young plants, made by small dark beetles that jump away like fleas when you get close. Bad damage can stunt seedlings.Tiny round shot-hole pits chewed in the leaves of young plants; the small dark beetles jump like fleas when disturbed. Heavy feeding stunts seedlings and slows establishment, the costly stage for a fast crop. Cause Flea beetles are small jumping beetles that chew on young cabbage-family leaves. They hurt seedlings most, because a tiny plant can't grow fast enough to recover.The crucifer and striped flea beetles chew the foliage of young brassicas; they overwinter in debris and field edges and are most damaging at the seedling stage and in spring, easing as the season warms, so fall crops face less of them. Organic treatment Cover young plants with a row cover, and if there are a lot of beetles, use a spray such as spinosad or a kaolin clay coating. The main goal is to get the seedlings big enough to handle it.Protect seedlings with row cover and spray spinosad or apply kaolin clay if pressure is high; the goal is getting plants past the vulnerable seedling stage. Prevention Cover seedlings with row cover from the start, keep them strong with steady water and food so they grow past the damage, and pull nearby weeds in the cabbage family.Use row cover from sowing or transplanting, keep seedlings vigorous with even water and feeding so they grow through the damage, clean up weedy brassica relatives, and favor fall plantings, which see lighter pressure.
Symptoms New transplants and seedlings wilt and grow badly or fall over. If you pull one up, you'll find small white maggots eating the roots. It is worst in cool, wet soil.Plants wilt, grow poorly, and may collapse; pulling a plant reveals small white maggots tunneling in the roots and lower stem. Damage is worst in cool, wet soil at spring establishment. Cause A small fly lays eggs at the base of the plant, and the maggots that hatch burrow down and eat the roots. The fly is most active in cool, damp spring weather.The larvae of the cabbage root fly (Delia radicum) tunnel into brassica roots; the adult lays eggs at the stem base, and the maggots feed below, sometimes killing young plants and opening the door to rots. Organic treatment Once the maggots are in the roots you can't save that plant, so pull and throw away the badly damaged ones. Put your effort into protecting the rest from new eggs.There is no effective rescue once maggots are in the roots; remove and destroy badly affected plants and protect the rest from further egg-laying. Prevention Put a small collar or disk around the base of each transplant to stop the fly from laying eggs there, or use a row cover. Avoid planting into cold, soggy soil, and rotate away from where cabbage-family plants grew.Place a collar or disk around the stem base at transplanting to block egg-laying, or use row cover; avoid planting into cold, wet soil and rotate away from where brassicas grew.
Symptoms Shield-shaped black-and-orange (or black-and-red) bugs on the leaves, which develop white or yellow spots, then brown and look tattered. It is mostly a warm-region problem and loves cabbage-family plants.Shield-shaped bugs up to about 3/8 inch, brightly black-and-yellow or black-and-red, feeding with piercing-sucking mouthparts that leave white stipples; affected leaves brown and look tattered, and heavy feeding wilts plants. A warm-region brassica specialist. Cause The harlequin bug is a colorful stink-bug relative that pierces the leaves and sucks the juices, leaving pale blotches. It strongly prefers cabbage-family plants, especially in the South.The harlequin bug (Murgantia histrionica) is a true bug that specializes on cole crops, most damaging in the southern United States, where it completes several generations a year and overwinters as an adult in debris and weeds. Organic treatment Search out and crush the bugs, their barrel-shaped striped eggs, and the young ones by hand. Use a row cover to keep them off, pull badly infested plants, and clear away weeds and old plants where they hide.Hand-pick and crush adults, nymphs, and the barrel-shaped egg rows; exclude with floating row cover; remove heavily infested plants. Insecticidal soap, pyrethrum, or neem help where pressure is severe. Prevention Clean up garden debris and cabbage-family weeds where the adults overwinter, use row cover, and consider a trap crop of mustard to lure them away, then destroy it.Eliminate overwintering sites by removing crop debris and crucifer weeds, use row cover, and deploy an early trap crop of mustard to divert overwintering adults, then destroy it before the main crop is set out.
Diseases
Symptoms Plants wilt in the afternoon, stay small and yellowish, and don't bounce back. If you dig one up, the roots are swollen into lumpy, club-shaped knobs.Plants wilt in the heat of the day, grow stunted and yellowed, and recover poorly; roots are swollen into distorted clubs and galls instead of normal fine roots, which later rot. Cause A germ that lives in the soil infects the roots of cabbage-family plants and makes them swell, which stops the plant from taking up water and food. It stays in the soil for years.A soilborne organism (Plasmodiophora brassicae) infects brassica roots and forms the galls, which block water and nutrient uptake. It persists in soil for many years, spreads in contaminated soil and water, and is favored by acidic soils below about pH 6.5. Organic treatment There's no cure once a plant has it, so pull and destroy infected plants. Keeping the soil closer to neutral helps hold it back, and growing cabbage-family plants only rarely in that spot starves the germ out.There is no cure once plants are infected; remove and destroy them. Raising soil pH toward neutral with lime suppresses the disease, and long rotations starve it out over time. Prevention Don't grow kohlrabi or other cabbage-family plants in the same spot more than once every few years, longer if you've had clubroot. Keep the soil from being too acidic, improve drainage, and clean soil off your tools.Rotate brassicas out of affected ground on a long cycle (up to 7 years where clubroot is known), keep soil pH up, improve drainage, and avoid moving infested soil on tools and boots.
Symptoms Yellow patches shaped like the letter V appear at the leaf edges, pointing inward, with black veins inside them. It can spread through the plant and rot it.Yellow, V-shaped lesions spread inward from the leaf margins, with the veins inside the V turning black; the disease moves through the vascular system and can blacken and kill the plant. Often the most serious brassica disease in warm, wet weather. Cause A bacteria that often comes in on infected seeds or transplants. It gets into the leaves and travels through the plant's veins, spreading fastest in warm, wet weather.A seedborne bacterium (Xanthomonas campestris pv. campestris) that enters through leaf pores and wounds and moves through the veins; it spreads in warm, wet conditions and on infected seed or transplants. Organic treatment There's no real cure, so remove and destroy infected plants to keep it from spreading. Don't handle or brush against the plants while they're wet.There is no effective cure; remove and destroy infected plants. Avoid working among plants when foliage is wet, since that moves the bacteria. Prevention Begin with clean, certified seed and healthy transplants, don't plant where cabbage-family crops grew recently, space plants for airflow, and avoid overhead watering.Start with clean, certified seed and healthy transplants, rotate 2 to 3 years away from brassicas, give plants room for airflow, and avoid overhead watering and handling wet foliage.
Symptoms Small, round, dark spots on the leaves, each with a yellow ring and faint target-like circles inside, that grow and run together in wet weather and turn sooty in the center.Small, circular, dark spots with a yellow halo and concentric rings that darken to a sooty center as spores build, enlarging and merging in wet weather. A foliar disease that weakens the plant. Cause Alternaria fungi that can travel on seed and survive on old plant debris, spreading in splashing water and needing wet leaves to take hold. Warm, humid weather brings it on.Alternaria fungi (chiefly Alternaria brassicae), which are seedborne and survive on crop debris, spreading in splashing water and needing leaf wetness to infect; warm, wet weather drives it. Organic treatment Pick off the worst leaves, water at the soil in the morning so leaves dry quickly, and clean up debris. Clean seed and airflow handle most of it; a copper fungicide can help in a wet spell if started early.Remove badly spotted leaves, water at the base in the morning so foliage dries, and clear debris. Clean seed and airflow are the main home levers; a copper fungicide can help in a persistently wet season if started early. Prevention Use clean seed, water at the base instead of overhead, give plants room so the leaves dry, and rotate crucifers and clear debris between plantings.Use clean seed, avoid overhead watering, space for airflow, and rotate crucifers and clear debris that carry the fungi between seasons.
Symptoms Pale yellow blotches on top of the leaves, with fuzzy grayish growth on the underside, mostly in cool, damp weather. Bad cases discolor the leaves.Pale yellow patches on the upper leaf surface with a fuzzy gray-white growth underneath, mainly in cool, damp weather; severe infection discolors and kills foliage. Kohlrabi's cool, humid season overlaps the weather it likes. Cause A mold-like disease that loves cool, wet, humid weather and spreads through the air and water. Wet leaves and crowded plants make it worse.A fungus-like organism (Hyaloperonospora parasitica) that thrives in cool, wet, humid conditions and spreads by spores in water and air; long leaf wetness and crowded plantings favor it. Organic treatment Open up the airflow so leaves can dry, remove the worst leaves, and use a copper spray if it's spreading. It's hard to stop once the weather stays damp.Improve airflow and let foliage dry, remove badly affected leaves, and apply a copper-based fungicide if pressure is high; the disease is hard to stop once humidity stays high. Prevention Space plants so air moves between them, water at the soil instead of from above, and pick resistant varieties if downy mildew is common where you live.Space plants for good air movement, water at the soil line rather than overhead, and choose resistant varieties where downy mildew is common.
Symptoms Pale, sunken spots dotted with tiny black specks appear on the stem and leaves, and the base of the stem darkens and rots, so the plant may topple. It often arrives on infected seed.Pale, sunken lesions peppered with small black fruiting bodies (pycnidia) on stems and leaves, and a dark, girdling canker at the stem base that can rot the crown and lodge the plant. Cause A fungus that comes in on infected seed or plants and attacks the stem and leaves of cabbage-family crops. It spreads in wet weather and lingers in old plant debris.The fungus Phoma lingam (Leptosphaeria maculans) is chiefly seedborne and survives in brassica debris; it spreads in cool, wet weather by splashing water and infects through wounds and pores. Organic treatment There's no cure, so pull and destroy infected plants. The real fix is prevention: clean seed, rotation, and clearing away old cabbage-family debris.There is no effective cure; remove and destroy infected plants. Management is preventive, since the pathogen is carried on seed and in residue. Prevention Start with clean, certified seed and healthy transplants, rotate away from cabbage-family crops for a few years, and clear away old brassica plants where the fungus survives.Use clean, certified, hot-water-treated seed and disease-free transplants, rotate 3 years away from brassicas, and bury or remove crop debris that carries the fungus over.
