Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June30, 1914, in cooperation with the U.S. Department of Agriculture. Andrew G. Hashimoto, Director/Dean, Cooperative Extension Service/CTAHR, Universityof Hawai‘i at Mänoa, Honolulu, Hawai‘i 96822. An equal opportunity/affirmative action institution providing programs and services to the people of Hawai‘i withoutregard to race, sex, age, religion, color, national origin, ancestry, disability, marital status, arrest and court record, sexual orientation, or status as a covered veteran.CTAHR publications can be found on the Web site <http://www.ctahr.hawaii.edu> or ordered by calling 808-956-7046 or sending e-mail to ctahrpub@hawaii.edu.

Plant DiseaseJuly 2005

PD-27

Morinda citrifolia, called noni in Hawaiian, is a sourceof food, dyes, and medicines esteemed throughout

the Pacific region. It once grew mainly in relatively pris-tine native forest and coastal ecosystems. Although stillabundant in natural habitats, noni is widely grown todayas an agricultural crop removed from its optimal environ-ment. Its successful cultivation today demands more knowl-edge and skill than was required of Hawaii’s indigenoushorticulturalists; they did not encounter the problems com-mon to modern agricultural soils that are contaminated witha wide range of introduced and invasive plant pathogens.

The most destructive soilborne pathogen of noni in Ha-waii is the root-knot nematode. These tiny, parasitic round-worms cause root infections and disease that severely un-dermine plant health, leading even to plant death. Dozensof acres in Hawaii planted to noni have been abandoned byfarmers in recent years due to root-knot disease, and theproductivity of other farms has been severely compromised.This condition and its management are described here.

Recognizing the symptomsAs the disease name suggests, diseased noni roots are char-acteristically galled, distorted, swollen, and knotty. The rootsurface may be cracked and wrinkled, with a corky texture,and rotten. Diseased roots are often discolored and irregu-lar in shape. Young lateral roots may have stubby, swollentips. Diseased root systems are sparse and diminished insize. A soft rot may develop inside severely affected roots.

Above-ground symptoms are usually the first abnor-malities noticed. Plants become stunted, with yellow leaves,sparse foliage, and unthrifty appearance. The leaves may“flag” (wilt or droop). These foliar symptoms resemblesome nutrient deficiency symptoms, of the elements ironand nitrogen in particular.

The bark of basal stems at ground level may crackopen and decay; plants with stems so girdled often die whenopportunistic or pathogenic fungi colonize the woundedstem tissues and rot the bark.

Noni Root Knot, a Destructive Diseaseof Morinda citrifolia in Hawaii

Occurrence of symptoms in fields may be patchy, withseverely affected, yellowed plants growing next to appar-ently unaffected plants. Symptoms can develop slowly overtime and take several years to cause a significant declinein plant health, or lead to plant death. Symptoms may beexacerbated by opportunistic or pathogenic fungi and bac-teria that colonize nematode-infected tissues.

Cause of noni root-knot diseaseSpecies of the plant-parasitic root-knot nematode(Meloidogyne species) cause this disease in Hawaii. Theseparasites damage noni roots by penetrating deep into tis-sues, and through their feeding they cause roots to swelland crack. In addition to this direct tissue damage, thesegalls attract fungi and bacteria that cause root discolora-tion and rot.

Disease distributionNoni root-knot disease occurs wherever noni is grown inagricultural soils in Hawaii, especially on land formerlydevoted to sugarcane, pineapple, or papaya. However, thedisease may also be introduced to any location where noniis grown from infected planting materials, including plantsderived from seeds or cuttings.

Disease cycleRoot-knot nematodes live in soils on plants. Meloidogynespecies have relatively broad host ranges. The members ofthis genus can infect more than 700 different plant hosts,including many common weeds and familiar vegetablecrops. They hatch from tiny eggs and move through the soilin search of food. They are so small (0.5 mm [2⁄100 inch] long)that they are virtually invisible to the unaided eye. Theyenter root tips and establish feeding sites deep inside theroots. Using a spear-shaped mouthpart, they withdraw nu-trients from root cells. Their life span is relatively brief, last-ing only several weeks. Each adult female will lay hundredsof eggs in the soil or in root tissues; when these hatch, the

Scot NelsonDepartment of Plant and Environmental Protection Sciences

UH–CTAHR Noni Root Knot, a Destructive Disease of Morinda citrifolia in Hawaii PD-27 — July 2005

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roots are reinfected. Nematode populations can become verylarge in soils where susceptible plants are grown continuouslyfor several years. For a crop such as noni, cultivated for manyyears, nematode populations can reach very damaging levels.

Root-knot nematodes do not move very far under theirown power. They usually are spread through the movementof infested soil, water, and plants. Their transport can occurthrough human activity including soiled hands, boots, equip-ment, and tools, or by carrying of soil and infested plantsfrom one location to another. They also spread by runningor draining water from irrigation and rainfall runoff.

Prevention and control of root-knot diseaseControl noni root knot by using integrated cultural and pre-ventive methods such as avoidance, sanitation, and appro-priate cropping systems.

Symptom recognitionLearn to recognize the foliar and root symptoms of noniroot knot and how to distinguish them from other diseasesor conditions. The only characteristic symptom is the rootgall. To diagnose the disease, brush away the top layer ofsoil and inspect the exposed, surface-feeding lateral rootsfor galls. The foliar symptoms mimic fertilizer deficien-cies (nitrogen, potassium, and iron) and are often the firstindication that there is something wrong with the roots.Periodically inspect plants in the field for symptoms. Nurs-ery operators should inspect plant root systems for infesta-tion before selling, distributing, or transplanting noni plants.

Disease-free nurseriesAvoid propagating noni in nematode-infested soil or me-dia. Start noni seedlings or cuttings only in sterile pottingmedia (e.g., cinder, peat, perlite, vermiculite) or heat-treated

soil. Do not use recently cultivated agricultural field soilfor starting seedlings or cuttings. Grow young noni plantson raised benches to prevent root contact with soil. Mini-mize water splashing during irrigation to reduce spread ofcontaminated soil or medium between pots. Media for plantgrowth may be treated with heat before use. Sufficient heat(at least 150°F for 15–20 minutes) is deadly to root-knotnematodes and may be achieved with steam, fire, electric-ity, or solar radiation (solarization).

Prevention and avoidanceAvoid planting noni in fields where crops susceptible toroot-knot nematodes have recently been cultivated. Pre-vent the introduction and establishment of root-knot nema-todes in an uninfested nursery or field. Inspect the roots ofyoung noni plants for root knot symptoms prior to trans-planting, and prevent their planting if infected—destroythem. Limit unnecessary foot or vehicle traffic into the farm.If a portion of a noni farm becomes infested with root-knot nematodes, perform all farm operations in that sec-tion last, rather than first, to minimize the dispersal of nema-todes on field implements, tractors, or vehicles.

Soil and media testingAssess soil or media for the presence and quantity of root-knot nematodes by submitting appropriate soil samples toa diagnostic lab or the CTAHR Agricultural DiagnosticService Center via your nearest Cooperative ExtensionService office. An appropriate soil sample consists of arepresentative composite sample taken from areas to beplanted; it should include the surface 6 inches of soil.

Preplant nematicidesPreplant soil fumigants, soil sterilants, or other approved

Advanced, above-ground symptoms of noni root-knot diseasemay include wilting and almost complete yellowing of leaves.This condition can lead to plant death.

Yellowing of noni leaves between veins is one of the first above-ground symptoms of noni root-knot disease. The pattern ofyellowing resembles iron deficiency (yellowing between veins).

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UH–CTAHR Noni Root Knot, a Destructive Disease of Morinda citrifolia in Hawaii PD-27 — July 2005

nematicides may be used to reduce initialnematode populations to non-damaging lev-els before planting. There are no postplantnematicides registered for use on noni in theUnited States.

SanitationSanitation involves decontaminating objectsused in noni cultivation. Tools (shovels),propagation materials (pots), clothing(boots), and farm vehicles (tires) may be asource of nematode infestation when theycome into contact with nematode-infestedsoil. Nematodes can be eradicated fromthese objects by cleaning them with a strongspray of water, followed by sterilizing witha 10% solution of household bleach.

Irrigation and fertilizer managementNoni plants may wilt due to nematode attackeven though soil moisture appears to be ad-equate. Because the damaged roots are soinefficient at the uptake of water and nutri-ents, some additional water and fertilizer mayreduce wilting or improve plant growth. Noniplants infected with nematodes respond well to foliar fertil-izers. However, it is best to avoid excessive irrigation andfertilizing of infected plants. Over-irrigating can help tospread root-knot nematodes and hasten root rot. Over-fertil-izing can lead to rotting of infected roots, followed by plantdecline and death.

Rogue diseased plantsRogueing involves removing diseased plants from the nurs-ery or field. This practice is best for new farms or whendisease is detected early. Rogue (pull up and burn or dis-card) diseased noni plants in areas or farms where newoutbreaks of root-knot disease are discovered. This willprevent or minimize the spread of nematodes from dis-eased plants to healthy plants along rows or between farmsand nurseries. If root-knot disease is already well estab-lished and severe, rogueing will not help to stop the dis-ease from spreading.

Composting and soil amendmentsOrganic composts and soil amendments deter nematodes.Preplant and postplant applications of organic soil amend-ments, composted plant materials (e.g., macadamia husks),and/or composted chicken manure provide some nematodecontrol while supporting good plant nutrition and encour-aging healthy plant growth. Organic amendments and com-posts favor biological control of root-knot nematodes by

providing a favorable environment for parasites and otherorganisms that compete with the nematodes. Avoid con-tact between mulches or composts and noni stems; the heatgenerated during composting can damage stem tissues andlead to rot and stem girdling.

Weed controlControl or eradicate all weed hosts of root-knot nematodeswhile cultivating noni and during fallow periods. Submitroot samples of weeds to a diagnostic laboratory or testingservice for nematode detection. Although visual inspec-tion of weeds for the characteristic root galls may be suffi-cient to determine their risk, some weed hosts may havemuch smaller galls than the swellings commonly observedon noni roots.

Appropriate cropping systemA diverse cropping system and crop rotations interrupt thespread, reproduction, and survival of nematodes. Considerusing a multicropping system combining noni with plantsthat are immune or highly resistant to root-knot nematodes.These plants may be interplanted with noni or used in croprotations. For example, sunn hemp (Crotolaria juncea),planted in alternating strips between rows or blocks of noni,may be effective for multicropping and crop-rotation sys-tems. Many of these nematode-resistant cover crops wouldmake excellent green manure crops or sources of compost

Young noni plant with advanced symp-toms of root-knot disease and fungalstem rot. The stem is completely girdledby the damage, and the roots are heavilygalled and rotten. This plant had nochance for survival.

After noni stems are girdled bynematodes and fungi, an internal rotcan develop that kills the plant. Herethe bark is partially cut away from anoni stem to reveal the vascular rotinside.

UH–CTAHR Noni Root Knot, a Destructive Disease of Morinda citrifolia in Hawaii PD-27 — July 2005

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for noni. Due to the wide host range of root-knot nema-todes, care should be exercised in the selection of com-panion crops for noni.

Host-free fallowNoni fields that are heavily infested with root-knot nema-todes should be placed in host-free fallow for a minimumof 1 year prior to replanting noni, preferably much longer—up to 3 years. Fallowing is not very effective unless weedhosts for Meloidogyne species are controlled.

Host resistanceAt present, no noni variety is thought to be resistant toroot-knot nematodes.

Abandon heavily infested fieldsIt may not be economically feasible to continue making farminputs in severely affected locations. In these cases, movingthe farm to a new location and using disease-free plantingmaterial may be the only practical solution to the problem.

Early symptoms of root-knot disease on young noni rootsinclude swellings, galls, unthrifty root growth and root rot. Eachof these tiny galls contains thousands of nematodes inside. Asthe roots grow, the galls become larger and roots crack open.

Diagnosing root-knot disease can be as simple as brushingaway the soil surface to reveal the galled and rotten roots.

The noni plant at right died as a result of root-knot nematodeinfection and stem girdling. The noni plant at left shows veryearly symptoms of root-knot infection (yellow leaves).

Noni lateral root exposed at the soil surface, showing grotesqueswelling and cracking caused by infection of the root by root-knot nematodes.

Basal stem is cracked open on this dying noni plant infectedwith root-knot nematodes. Root galls are also visible.