(now R. citrophilus) was reported as the primary cause.
Hosts
Wide host range. Over 200 known hosts; very important pest of bananas and citrus.
Distribution
Widespread in tropical and subtropical regions of the world (West Africa, Central and South America, Hawaii, Florida, Puerto Rico, Cuba, Australia).
Life Cycle
Migratory endoparasite spending its adult vermiform life in the root, but capable of emerging in adverse conditions. At 24-32 C the life cycle took 20-25 days. 18-20 days at 24-27 C. All larval stages and females are infective. Fertilization is usual but parthenogenesis does occur. Eggs hatch occurs in 3-7 days. Female produces an average of about 2 eggs/day (range is 0.5-6 eggs/day/female). Male does not penetrate intact roots and may not feed. Species survives less than 6 months in soil free of host roots. At least 2 biotypes have been demonstrated. They are similar morphologically but differ in host range. The "banana race" attacks banana but not citrus. The "citrus race" is pathogenic to both citrus and banana. In 1984, the citrus and banana race were described as sibling species with different chromosome numbers. There may also be a sugarcane race. R. similis - original banana race - banana, but not citrus. R. citrophilus - citrus burrowing nematode- citrus and banana. R. similis - n=4 R. citrophilus - n=5.
Symptoms-Pathogenicity
Feeds in cortex, resulting in lesions and cavities, root breakdown, secondary decays; e.g., Fusarium oxysporum and Rhizoctonia solani in banana. Seldom feeds in vascular tissue. Causes "Blackhead" or toppling disease of bananas - root system reduced and weakened so that tree falls under weight of fruit or in wind - total crop loss. May reduce vigor of sucker growth for new trees and delay rate of fruit development. Symptoms of toppling disease: Initial entry of the nematode into the root produces a reddish, elongate fleck parallel to the root axis. The fleck or discolored area enlarges as the nematode and progeny feed. The older parts of the lesion turn black and shrink, with the advancing margin remaining red. Neither the nematode nor the eggs are found beyond the red margin; they are also rare in the older portions of the lesion. Continued feeding causes extensive, deep lesions on roots and rhizomes. Uninfested bananas in Central America yielded 17,000lbs/acre more fruit than infested.
Infestation causes decline of citrus in Florida; 40-70% yield decrease in oranges and 50-80% in grapefruit. From surface 2 1/2 feet, 30% of feeder roots destroyed; below that, 90% of roots destroyed, leaving less than half of functional feeder roots. Especially important on non-organic sands in central Florida (less than 1/4% OM). These sands have a moisture holding capacity of 5-7%, and permanent wilting occurs at 2 1/2%. Roots in moist, deeper soils are 90% destroyed, and upper soil is dry for 5 months, so trees become stressed. Nematodes can be found up to 12 feet below the soil surface in citrus groves. Above-ground symptoms usually appear 1 year after initial infection of roots (the length of time required for population increase and extensive root injury). Parasitized, but healthy-appearing trees occur 1-3 rows in advance of those visibly declined. Leaves from infected trees contain less K and N than healthy leaves. Heavy fertilization with KCl causes improvement in K levels. Some 15,000 acres are seriously infested, but this represents about 1% of Florida citrus, and some say too much attention has been paid to this nematode; however, this attention may have reduced spread and protected other regions from infestation. Millions of black pepper trees have been lost in Indonesia (approx. 90% of the crop) to "Yellows disease" caused by R. similis. Nematode also is a pest of sugarcane in Hawaii.
Management
Strategies for Banana: Start/replace with clean stock; leave soil fallow for 6 months with no root fragments; flood soil; use preplant nematicides and nematicides at planting (will approximately double yield from 22.5 metric tons/hectare to 40 metric tons/hectare); postplant application of nematicides will increase yield by 40% (from 39 metric tons/hectare to 51.86 metric tons/hectare); clean or pare infected tissue from rhizome; use hot water dips (55 C for 20 minutes). In India a common practice is to coat the corm in clay soaked with nematicide. Propping or guying of trees prevents total loss due to
toppling, but the practice is expensive. A healthy plantation may remain productive for up to 20 years; an infested one for only 2 or 3 years. Thus, there is considerable economic benefit to management.
Strategies for citrus: Push and treat, 60 gal telone/acre; 2 year weed-free fallow. Hot water dips of nursery stock (50 C for 10 minutes); creating buffer zones around infested orchards (15-20ft); deep ripping to prevent root transfer; using high rates of nematicides. Also, plant resistant citrus vars: milam, carigan, rough lemon, Ridge Pineapple - Citrus sinensis, but none are immune.
Importance
Quarantine pest in California; frequently intercepted on ornamentals in nurseries.
Characteristics
Radopholus - Small nematodes, adults less than 1 mm long, body straight or slightly curved ventrally when killed by heat. Marked sexual dimorphism in anterior region. Female with low, rounded, lip region with moderate head skeleton, continuous with or slightly offset from body; stylet short (14-23 um) stout, with well developed basal knobs; median esophageal bulb well developed; esophageal glands in a long lobe overlapping the intestine dorsally; vulva usually just post median with paired opposed gonads, occasionally more posterior with single anterior gonad and post vulval sac. Tail elongate, conoid, 5-7 anal body widths long. Male lip region high and rounded, knob-like, offset, stylet weak without basal knobs; esophagus degenerate without prominent median bulb; tail elongate conoid with elongate bursa reaching almost to tail tip.
References
- Evans, K., D.L. Trudgill, and J.M. Webster. 1993. Chapter 1. Extraction, Identification and Control of Plant Parasitic Nematodes. in Plant Parasitic Nematodes in Temperate Agriculture. CAB International, UK. 648 pages.
- Orton Williams, K.J. and M.R. Siddiqi. 1973. Radopholus similis. C.I.H. Descriptions of Plant-parasitic Nematodes. Set 2, No. 27. Commonwealth Institute of Parasitology. C.A.B. International. 4 pages.
- Maggenti, A.R. 1981. General Nematology. Springer-Verlag, New York. 372 pages. (page 187).