Radopholus citrophilus - burrowing nematode / R. similis Rev 10/96 Morphology and Anatomy: Similar to Pratylenchus in general shape and size, but has 2 ovaries. Lip region is conical; tail tip is acute and rounded. Long dorsal overlap; male with bursa. Sexual dimorphism - male is slender and may have reduced stylet and high, rounded lip region. Distribution: Widespread in tropics, West Africa, Central and South America, Hawaii, Florida, Puerto Rico, Cuba, Australia. Economic Importance: A-rated pests in California; frequently intercepted on ornamentals in nurseries. Nursery imports sometimes have been routed through Texas or Mexico from Florida. There is a current initiative in California to eliminate the burrowing nematode quarantine, promoted by California Assoc. of Nursermen and partially supported as a cost-cutting measure by the regulatory branch. Feeding Habit: Migratory endoparasite. Hosts: Over 200 known hosts; may be most important nematode pest on fruit crops (especially citrus and banana) in the tropics. Races or biotypes have been recognized, and, in 1984, the citrus and banana race were described as sibling species with differen chromosome numbers (Huettel, Dickson and Kaplan, 1984. Proc. Helm. Soc. Wash. 51: 32-35). 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 There is also evidence of reproductive isolation in mating tests. Considerable consternation among regulatory nematologists in Florida who argue that not enough populations have been studied, and that routine separation is very difficult. Life Cycle: Grapefruit roots - Eggs hatch occurs in 3-7 days; life cycle completed in 18-20 days at 24-26 C. (fast for a plant-parasitic nematode); life cycle extended at lower temperatures. Female produces an average of about 2 eggs/day (range is 0.5-6 eggs/day/female). Males are present, but female can produce eggs without fertilization - parthenenogenetic capability. Male does not penetrate intact roots and may not feed. The number of individuals in roots is not particularly high (50 to 120 nematodes/gram of root). Species survives less than 6 months in soil free of host roots, but removing all host roots may be difficult to accomplish. Damage: 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 discolored area 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 (D-D at 300 l/hectare will approximately double yield from 22.5 metric tons/hectare to 40 metric tons/hectare); postplant application of nematicides (DBCP at 25 l/hectare in Oct. and 15 l/hectare the following March) 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); use DBCP dips (causes some phytotoxicity). 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 program (Florida), 60 gal telone/acre; 2 year weed-free fallow. Formerly used ethylene dibromide (ethylene dibromide (EDB)) at 25 gal/acre every 6 months. The following methods are recommended by Florida Dept. of Ag. (process about 15,000 samples/year): 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. Over 300 miles of such buffer. Recent efforts have focused on trenching and the use of plastic barriers. Also, plant resistant citrus vars: milam, carigan, rough lemon, Ridge Pineapple - Citrus sinensis, but none are immune. Kaplan (Journal of Nematology, 1984, 1985 worked with Carrizo citrange rootstock (Citrus sinensis X Poncirus trifoliata) - variously reported as being resistant and tolerant. Tried range of sources, with enormous variations. Then, created twin plants from the same seed - should be clones in absence of somatic mutation - but this produced equally high variation. Clearly, the resistance mechanism is not understood. If nematode population is held at 1/g root with nematicides, little damage occurs, but population tends to explode, possibly due to flush of new feeder roots. It requires 3 applications of vydate (oxamyl) per year to hold nematode populations to this level. The alternative is to "accept" root damage, and to irrigate and attempt to increase yield by 2 boxes fruit/acre (about $500) (Note - Florida citrus not usually irrigated). O'Bannon feels that improved orchard management can reduce need for "push and treat" programs, except at very high nematode population levels. Additional Information: R. similis first reported from banana in the Fiji islands in 1893. Spreading decline of citrus was first recognized as a disease in 1930, but it was not until 1953 that R. similis (now R. citrophilus) was reported as the primary cause.