The infective stage that emerges from the egg is the second-stage juvenile, the first moult occurring within the egg. It locates and penetrates a root and establishes a feeding site, usually within the pericycle and vascular tissues. The oesophageal gland secretions injected into the root cells incite the production of multinucleate giant cells (syncytia, nurse cells, transfer cells) which provide a continuous source of food for the parasite. A gall is formed at the feeding site due to extensive hypertrophy and hyperplasia of the root cells. The juvenile feeds, grows and becomes sausage-shaped. It undergoes three further moults, the third and fourth moults occurring within the cuticle of the second moult. The adult female is spheroidal in form and after about 15-30 days of feeding and growth starts laying a large number of eggs in a gelatinous matrix secreted by the rectal gland cells through the anus, located next to the vulva at the posterior end. Large egg-masses are produced within and outside the induced galls. Males are vermiform and probably do not feed. During adverse conditions most of the juveniles may develop into males.
Damage to the host is largely due to the disruption of vascular tissues and extensive hypertrophy and hyperplasia of root cells which prevents, or reduces, the uptake of water and nutrients. Besides the knotted, or galled, roots, an infected plant shows poor growth, nutrient deficiency symptoms (eg. chlorosis), general wilt symptoms and poor yield. The damage can often be aggravated by the parasites interaction with other micro-organisms such as fungi and bacteria.
As a result of the differential host range of the different species of Meloidogyne, accurate identification is essential for the control of most Meloidogyne species particularly where more than one crop is grown and where their control is planned using rotation of different susceptible and resistant crops. As the taxonomy of the group is complex, species being difficult to distinguish using classical methodologies, alternative techniques such as PCR based methods are gaining acceptance.
The image above is of Meloidogyne decalineata galls on Coffea arabica from Kilimanjaro, Tanzania. Photo provided by John Bridge, IIP.