NC State Watermelon Disease Handbook

Root-knot Nematodes (Meloidogyne spp.)

Disease:

Root-knot Nematodes

Cause (pathogen):

Meloidogyne spp., Meloidogyne incognita, M. javanica, M. arenaria, M. hapla.

Symtoms:

Plants affected by root knot nematodes usually first appear in clusters on side-hills of fields with sandy soils. Plants whose roots are infected shortly after transplanting will appear stunted throughout the season. Plants infected later in the season normally wilt as soon as they are exposed to the slightest water stress. The diagnostic feature of root knot nematode infection is the presence of galls on roots of affected plants. Severely affected plants may have galls as large as 1 inch in diameter.

Source of primary inoculum:

Hard-shell nematode eggs that can survive in soil indefinitely.

Source of Secondary inoculum:

None

Spread:

Infested plant debris and mechanical spread with soil on farm implements.

Control:

Since nematode eggs can survive indefinitely, root knot nematodes always will be a threat in fields with a history of the disease. Long rotations with grain crops may help maintain reduced nematode population levels. Chemical control using fumigants or nematicides are necessary for watermelon production in fields with a history of root knot problems.The best control mechanism is to use resistant varieties.

Disease cycle:

The mature female root knot nematode is a sedentary endoparasite. Eggs deposited by the mature female will persist in the soil for several years. Young second stage larvae enter roots from the soil and remain there until maturity with the production of new egg masses. The nematode disease is more severe on sandy soils.

Source:

K. R. Barker, Plant Pathology Department, North Carolina State University,, Raleigh, NC, 27695-7616.

Storage:

Root-knot nematodes must be maintained on live plant material, since they are obligate parasites. It is necessary to reculture them every 70-90 days. Usually they are maintained in the greenhouse on 'Rutgers' tomato.

Inoculum Increase:

To maintain pure cultures, egg masses are picked from roots of infected plants and placed into a small beaker (50 mL) containing tap water. The beaker is then placed under a dissecting microscope to make sure eggs are viable. Ten to fifteen egg masses are placed under a 'Rutgers' tomato plant as it is being transplanted into a moist, sterilized 1:1 sand:soil mixture. Another method that can be used is inoculating plants with eggs that were extracted from roots using NaOCl (described later). Usually takes 3-4 months before inoculum is ready to use.

Inoculum Preparation:

Inoculum is prepared according to the method described by Hussey and Barker (1973). Inoculum is prepared by first cutting infected roots of a given species/race into 2-4 cm pieces. A 1% NaOCl solution is then poured over roots and stirred for four minutes. This solution is poured into a 40-mesh sieve located over a 10-liter bucket and rinsed with cold tap water, thus washing eggs into the bucket. The bucket is partially filled to obtain 2.5 liters of solution. Then the water and eggs in the bucket are poured into a 500-mesh sieve while vibrating the sieve against the side of the sink. The 500-mesh sieve was then turned over and eggs were washed back into the bucket and filled with 2.5 liters of water. After doing this twice, eggs were washed to one side of the sieve and washed into a 1-liter plastic beaker. This process of preparing eggs to use as inoculum is done several times to obtain enough eggs.

Quantification:

The number of eggs contained within the solution was determined by taking a 5-ml sample of eggs from the well-agitated solution in a 1-liter beaker, and placing it into 95 ml of tap water. Five ml of this solution is decanted into a counting dish and placed under a dissecting microscope. The number of eggs contained within a half-dish are counted and then multiplied by 8000 to get the total number of eggs in the stock solution.

Inoculum Distribution and Delivery:

Seeds must be planted into a 1:1 soil:sand mixture. Plants are ready to be inoculated when they reach the 2 to 3 true leaf stage. The inoculum must be standardized (200 eggs/ml) to allow each plant to be inoculated with 25 ml of solution for a total of 5000 eggs. This is accomplished by placing 1 million eggs in 5 liters of water. Twenty-five ml of solution is removed from the bucket and placed on the soil around the base of each plant (1 plant/pot). The solution must be kept well agitated to prevent the eggs from falling to the bottom of the solution during inoculation. After all plants are inoculated, 13 mm of a 1:1 moist, sterile sand:soil mixture is placed on top of the soil in each pot to protect eggs from desiccation.

Host:

Citrullus lanatus, watermelon.

Source of Resistance:

Not reported.

Susceptible check:

Tomato cv. Rutgers

Disease Response:

First symptoms are apparent 5-7 days after inoculation; however, these symptoms are below ground. After 10-12 weeks rate plants individually according to the gall index rating system. This is a system that rates roots by the percentage of the root that are galled, varies from 0% to 100%.