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Symptoms and Signs

Most soilborne plant pests and diseases are not evident aboveground until they are well established. Early symptoms caused by root feeding pests are due to impaired water and nutrient uptake. These symptoms include stunted plant growth, decreased vigor and yield, premature leaf drop and an increased tendency to wilt or dieback during dry periods. Radopholus similis causes a slow decline of many plant species, but symptoms are distinctive in banana and plantain (Musa hybrids and cultivars.), citrus (Citrus spp.) and black pepper (Piper nigrum)

Banana
The most dramatic disease symptom in banana plantations is the uprooting (toppling) of plants (Figure 2). Burrowing nematode feeding destroys anchor roots (Figure 3) and makes plants susceptible to toppling, especially when fruiting or during strong winds. Additional aboveground symptoms in bananas and plantains caused by root damage include slow sucker formation, delayed fruiting, smaller fruit and reduced bunch weight, and a shortened plant life. Radopholus similis kills feeder roots and creates reddish-brown lesions on larger root surfaces, both at the point of entry and throughout the cortex (Figure 4). Nematodes do not invade the central cylinder (stele), but heavy infestations can girdle and destroy roots (Figure 5). Eventually, burrowing nematodes can migrate from roots into the rhizome causing black, circular lesions, hence the name blackhead disease (Figure 6). Emerging roots may be attacked as they grow out of the infected rhizome.

Figure 2	Figure 3	Figure 4
Figure 5	Figure 6	Figure 7

Citrus
Spreading decline was first reported from Florida in 1928, but R. similis was not identified as the causal agent until 1953. The decline spreads quickly from tree to tree, with thinning foliage in the upper canopy as an early disease symptom (Figure 7). Leaves and fruits are small, the latter dropping early, as branch ends become bare and eventually die back. Root damage is most severe below 25 to 30 cm (10 to 12 inches) and shallower roots may appear unaffected. In the deep sandy soils of Florida, half of the feeder roots might grow 1 to 6 m (3 to 20 feet) deep and be destroyed, causing trees to wilt quickly as soil moisture decreases. Dark lesions appear at points of entry on feeder roots and expand as nematodes burrow through the root cortex. Roots become stunted, with irregular swellings, due to root tip damage and tissue hyperplasia (abnormal increase in cell number).

Black pepper
The main symptoms of yellows, or slow wilt disease of black pepper, are pale yellow leaves that droop and then fall from the vine. Other symptoms caused by decreased water and nutrient uptake are slow plant growth, flower drop, and vine dieback. Symptoms are more pronounced during dry periods, but if moisture becomes available early in the disease (e.g. tropical monsoon rains) leaves are replaced and vines appear to recover (Figure 8). In 3 to 5 years, however, the disease will reemerge, and so the name ‘slow wilt’. Burrowing nematode attacks both young and old plants, so vines replanted in infested soil normally die within 2 years. Thin, white feeder roots have purplish lesions and are quickly destroyed. Lesions are harder to see on older, brown roots, which are slower to rot.

Morphology of R. similis
The signs (visible presence of the pathogen) of these diseases are the various stages of R. similis observed in soil and plant root samples. All nematode stages are vermiform (wormlike), colorless and less than 1 mm in length. Adult males and females are different in appearance (sexual dimorphism), the males having poorly developed stylets and a knob-like head caused by an elevated, constricted lip region (Figure 9). Both males and females have long, tapered tails with rounded or indented ends (Figure 9). The male has a sharp, curved spicule (male reproductive organ), enclosed in a bursa, or sac (Figure 10). Females are between 550 and 880 µm (0.55 to 0.88 mm) in length and about 24 µm in diameter (Figure 11), with well-developed stylets 16 to 21 µm (average 18 µm) long (Figure 12). Males are smaller than females, 500 to 600 µm in length (Figure 13). Juveniles are often present in both root and soil samples and average between 315 to 400 µm in length with stylets 13 to 14 µm long.

Figure 8	Figure 9	Figure 10
Figure 11	Figure 12	Figure 13

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