Pests
Symptoms Lots of tiny round shot holes peppering the leaves, made by tiny dark beetles that jump like fleas when you come near. Arugula is one of their favorites, and heavy damage can riddle or kill young seedlings.Small round shot-holes pepper the foliage and the tiny dark beetles jump when disturbed. Arugula and other mustards are highly preferred hosts, so pressure is often intense; seedlings can be stunted or killed and the holes mar the harvested leaves of a crop eaten raw. Cause Flea beetles are small jumping beetles that chew little pits and holes in cabbage-family leaves. They are drawn strongly to arugula and mustard and hit small seedlings hardest, since a tiny plant cannot outgrow the damage.Crucifer and striped flea beetles (Phyllotreta species) chew the foliage of brassicas and strongly prefer arugula and mustard; adults overwinter in debris and emerge to attack the earliest, smallest plants. Multiple generations occur through the season. Organic treatment Cover young plants with a light row cover from the day you sow, and if beetles are heavy use a spinosad spray or a kaolin clay coating. The main goal is to get the seedlings up and big fast, before the beetles riddle them.Floating row cover from sowing is the most effective tool; spinosad or kaolin clay help under heavy pressure. The aim is rapid establishment past the vulnerable seedling stage, so push fast growth. Prevention Use row cover from sowing (the single best defense), keep seedlings growing fast with steady water, sow when beetles are less active, and clear nearby cabbage-family weeds. Some growers plant a sacrificial arugula patch as a trap crop.Exclude beetles with row cover from sowing, keep seedlings vigorous with even water and a light feed, clean up crucifer weeds and crop debris, and consider an early trap planting of arugula or mustard to divert beetles from the main crop.
Symptoms Clusters of tiny soft green or gray insects on the undersides of leaves and on new growth, sometimes with curled, sticky leaves. The waxy gray-green cabbage aphid hides deep in the crinkles and is hard to wash out.Colonies of green peach aphids and the waxy gray-green cabbage aphid (Brevicoryne brassicae) on leaf undersides and tips distort the foliage; honeydew and the aphids lodged in the leaves contaminate a raw-eaten crop, and several aphids vector viruses. Cause Aphids are small insects that suck sap from the plant. They breed quickly, especially on soft growth from too much fertilizer, and they can spread plant viruses; the cabbage aphid in particular loves brassicas like arugula.Several aphid species feed on arugula; they reproduce rapidly, favor the soft growth excess nitrogen produces, and the cabbage aphid is especially hard to dislodge from sheltered leaf folds. Organic treatment Blast them off with a strong spray of water, or use insecticidal soap or neem oil, hitting the undersides of the leaves. Repeat every few days until they are gone, and pick before a colony lodges in the leaves.Dislodge colonies with a strong water jet or spray insecticidal soap or neem, covering leaf undersides; repeat every few days. Conserve ladybugs, lacewings, and parasitoid wasps. Prevention Welcome helpful bugs like ladybugs and lacewings, go easy on nitrogen, check new growth often, and use row cover, which also keeps out flea beetles.Scout growing tips early, avoid overfeeding nitrogen, conserve natural enemies, and exclude with row cover; harvest the quick crop before colonies build.
Symptoms Ragged chewed holes in the leaves, green caterpillars on the plants, and dark droppings. Because you eat the leaves, even a little chewing is a problem.Ragged feeding holes with green or striped larvae and frass; imported cabbageworm, cabbage looper, and diamondback moth larvae feed on arugula, and on a fast leaf crop even light feeding downgrades the harvest. Cause Several moths and white butterflies lay eggs on cabbage-family leaves, and the caterpillars that hatch (imported cabbageworms, loopers, and small wriggling diamondback larvae) chew the foliage.The larvae of several brassica moths and butterflies feed on the foliage; the adults lay eggs on the leaves, and warm-region plantings see the most pressure. Organic treatment Spray Bt, which is safe and targets only caterpillars, or pick them and their eggs off by hand from the undersides of leaves. Spray again after rain, since it washes off.Apply Bt (Bacillus thuringiensis) or spinosad targeting young larvae and hand-pick where practical; repeat after rain. Prevention Use a row cover to keep the adults from laying eggs, clean up old cabbage-family plants and weeds, and check leaf undersides for eggs and small caterpillars.Exclude egg-laying adults with row cover, clean up brassica debris and weed hosts, and scout leaf undersides for eggs and early instars.
Symptoms Ragged holes with smooth edges chewed in low leaves, slimy trails on the leaves and soil, and worst damage in cool, damp, shady spots and after rain.Smooth-edged holes in low foliage with silvery slime trails, worst in cool, wet, densely planted or heavily mulched beds; a tender low salad green like arugula is highly susceptible. Cause Slugs and snails are soft, moisture-loving mollusks that feed at night and rasp holes in tender low greens like arugula. They thrive in damp mulch and dense, shaded plantings.Several slug and snail species feed nocturnally, rasping the foliage; they shelter in moist debris and mulch by day and build up in cool, wet conditions. Organic treatment Hand-pick at night or after rain, set out shallow traps, and use an iron-phosphate slug bait, which is pet-safe. Reduce damp hiding places and water in the morning so the surface dries by night.Hand-pick after dark or rain, use shallow traps and iron-phosphate baits (pet-safe), and remove daytime shelter; morning watering lets the surface dry before they feed. Prevention Thin and space plants for airflow, keep mulch light near the plants, clear damp debris, and water early in the day rather than at dusk.Reduce moisture and shelter near the plants: thin for airflow, keep mulch off the crowns, clear debris, and irrigate in the morning rather than the evening.
Diseases
Symptoms Pale yellow or angular patches on the tops of the leaves with a fuzzy grayish-white mold on the undersides, mostly in cool, damp, crowded conditions. Bad cases discolor and collapse the leaves.Angular yellow-to-brown lesions on the upper leaf surface with grayish-white sporulation beneath, spreading in cool, wet, humid weather; lesions brown and the leaves collapse. It is a leading arugula disease. Cause A mold-like disease that loves the same cool, wet weather arugula grows in, spreading through wet leaves and splashing water. Crowded plants and long leaf wetness make it worse.Hyaloperonospora parasitica (the brassica downy mildew oomycete), favored by cool temperatures, long leaf wetness, and high humidity; it spreads by wind- and splash-borne spores and survives on debris. Organic treatment Remove the worst leaves, open up the airflow so leaves dry faster, and water at the soil instead of over the plants. It is hard to stop once it gets going, so prevention matters most.Cultural control dominates: improve airflow, avoid overhead irrigation, and remove infected leaves and debris; there is no rescue once it is established in humid weather. Prevention Space plants for good airflow, water at the base, choose resistant varieties where available, and rotate where you plant arugula and other brassicas.Widen spacing for airflow, use drip rather than overhead water, choose resistant cultivars where offered, and rotate away from brassicas for several years.
Symptoms White, blister-like bumps on the undersides of the leaves, with yellow spots on top. Badly spotted leaves are not good to eat.White, blister-like pustules on leaf undersides with chlorotic spotting above; severe infection distorts and downgrades the foliage. A common brassica-greens disease, worse in the South. Cause A different mold-like disease, not a true rust, that makes white pustules on cabbage-family leaves and spreads in cool, moist weather. It can ruin a leafy crop quickly.The oomycete Albugo candida, favored by cool nights, mild days, and leaf wetness; it spreads by splash- and wind-dispersed spores and survives on debris and as oospores. Organic treatment Remove and destroy infected leaves, improve airflow, and water at the soil line. As with downy mildew, prevention works better than trying to cure it.Remove infected leaves and debris, improve airflow, and avoid overhead irrigation; chemical options are limited for the home garden. Prevention Space plants out, avoid wetting the leaves, choose resistant varieties where it is common, and rotate your planting spot.Grow resistant cultivars where white rust is endemic, space for airflow, use drip irrigation, and rotate away from brassicas.
Symptoms Seeds that never come up, or seedlings that flop over at the soil line with a pinched, rotted stem and then die. It strikes in cold, wet soil.Poor stands from seed and pre- or post-emergence collapse of seedlings with water-soaked, pinched stems at the soil line, worst in cold, wet, poorly drained soil where germination is slow. Cause Soil fungi attack the seeds and tiny seedlings, especially when the soil is cold and soggy and the seeds are slow to come up.Soilborne Pythium, Rhizoctonia, and Fusarium species attack germinating seed and seedlings; cold, saturated soil that slows emergence favors them. Organic treatment There is no cure for an affected seedling, so focus on prevention. Pull and discard collapsed seedlings.No cure once seedlings collapse; manage by prevention and remove affected plants. Prevention Do not sow into cold, soggy soil, plant shallowly in well-drained ground, do not overwater, and use fresh seed so it comes up fast.Sow shallowly into well-drained soil when temperatures favor quick emergence, avoid overwatering, and use fresh viable seed.
Symptoms Small dark spots on the leaves that grow into round patches, sometimes with rings like a tiny target or a pale halo. Spotted leaves are not good to eat.Small dark lesions enlarge into circular spots, Alternaria often with concentric rings (a target look) and Cercospora as gray-brown spots with a halo; on a leaf crop even moderate spotting reduces the harvest. Favored by warm, wet conditions. Cause Fungi that spread in warm, wet weather and infect the leaves, leaving spots. They are worse on stressed plants and in crowded, damp plantings.Alternaria (A. brassicae, A. brassicicola) and Cercospora fungi infect brassica foliage, spreading by spores in wind and splashing water; infected seed, crop debris, and long leaf wetness drive them. Organic treatment Remove and destroy spotted leaves, improve airflow so leaves dry quickly, and avoid wetting the leaves when you water. Start with clean seed.Remove spotted leaves and debris, improve airflow, and avoid overhead watering; a copper-based fungicide can help under heavy pressure. Prevention Use clean seed, rotate away from brassicas, space plants for airflow, water at the soil line, and clear crop debris.Use clean, disease-free seed, rotate away from brassicas, space for airflow, water at the soil line, and clear crop debris that harbors spores.
