of leaves yellow, 5 = about 50 percent of leaves yellow, and 

 9 = all leaves yellow. Plants scored 1 to 3 are classified as 

 resistant. 



Levels of resistance to leafhopper yellowing among culti- 

 vars in a test can be expressed as percentage of resistant 

 plants or as ASI. Resistant and susceptible checks should 

 be included at least once per replication. A single score may 

 be sufficient with a severe infestation of nymphs (48). Infes- 

 tation levels can be compared among studies when data are 

 obtained as nymphs per gram of dry matter (45). 



Root-Knot Nematode Resistance 



J. H. Elgin, Jr. 



Beltsville Agricultural Research Center, Beltsville, Md. 



Methods used at the Irrigated Agriculture Research and Ex- 

 tension Center, Prosser, Wash.— The most satisfactory 

 method for evaluating resistance to root-knot nematodes 

 (Meloidogyne hapla Chitwood, M. incognita Chitwood, and 

 M. javanica (Treub) Chitwood) uses nematode larvae applied 

 in an aqueous suspension (34, 75). Evaluations should be 

 conducted in either a greenhouse or a growth chamber. Op- 

 timum temperature for tests with M. hapla is 25°C (33); op- 

 timum temperature for tests with M. incognita and M. 

 javanica is 30°. A 12- to 16-h photoperiod is recommended 

 for tests with all three nematode species. Seeds of test 

 cultivars are planted in rows in flats filled with steamed or 

 fumigated soil. A spacing of 3 cm between rows with seeds 

 planted 1 to 2 cm apart within rows is optimum. A minimum 

 of 100 plants per cultivar is recommended, divided over at 

 least four replications. Resistant and susceptible check 

 cultivars are included in each test. 



Two weeks after planting, an aqueous suspension of active 

 root-knot nematodes is distributed over each flat at the rate 

 of 600 nematodes per seedling (18). Twelve weeks later, the 

 plants are carefully dug and their roots washed and exam- 

 ined for root galls. Each plant is rated on a scale of 1 to 4: 

 1 = no galls, 2 = 1 to 10 galls, 3 = 10 to 100 galls, and 

 4 = > 100 galls. Care must be used when digging the plants 

 to avoid excess root breakage and loss of galls. 



Root-knot nematode resistance among cultivars can be 

 expressed as percentage resistant plants or as ASI. Only 

 class 1 plants are considered resistant when calculating 

 percentage resistant plants. Any galling is considered a 

 susceptible reaction. Cultivar reations are compared by sta- 

 tistical analyses. M. hapla resistance ratings for 179 culti- 

 vars were reported by Elgin and others (19). 



Nematodes for evaluating and screening studies are cul- 

 tured on roots of tomato plants in pots in the greenhouse 

 (other susceptible hosts also may be used). Nematodes are 

 extracted from the tomato roots by discarding the tops, 



washing the infected roots in water until free of all soil, and 

 disinfecting the root surface in 10 percent chlorox for 5 min. 

 The roots are placed on a 20-mesh sieve screen, the screen 

 is set in a pan of water, and the water level is raised slightly 

 above the screen surface. Newly hatched nematode larvae 

 begin emerging in less than 24 h, settle into the water below 

 the screen, and continue emerging for 7 to 21 days (or 

 longer). The water is changed daily and the nematodes are 

 stored in a refrigerator at 3°C until they are used. Because 

 nematodes settle in standing water, they can be concen- 

 trated by siphoning off excess water. Nematode concentra- 

 tion can be determined by counting the nematodes in sev- 

 eral 1-ml samples, using a stereomicroscope. Nematodes 

 should be used as soon as possible after extraction to avoid 

 lowering their infectivity. 



A less precise, yet often useful, way of inoculating with root- 

 knot nematodes is to shred infected tomato roots and mix 

 them with the potting soil (5, 44, and 82). Seeds of test en- 

 tries are then planted in the soil. A criticism of this method 

 is that hot spots of inoculum often occur, and seedling 

 infection may not be uniform. For preliminary work or gen- 

 eral cultivar comparisons, however, this method of inocula- 

 tion may be more appealing than extracting the nemtodes. 



Stem Nematode Resistance 



J. H. Elgin, Jr., B. D. Thyr, and B. J. Hartman 

 Beltsville Agricultural Research Center, Beltsville, Md., 

 and Univeresity of Nevada, Reno 



Methods used at Irrigated Agriculture Research and Exten- 

 sion Center, Prosser, Wash., and University of Nevada, 

 Reno.— Smith (80) was the first scientist to compare levels 

 of stem nematode (Ditylenchus dipsaci (Kuhn) Filipjev) 

 resistance in several alfalfa cultivars. Other reports about 

 screening have been published since (6, 16, 31, 32, 35, 38, 43, 

 79, and 90). Resistance ratings for 179 cultivars were 

 reported by Elgin and others (19). Generally, techniques that 

 use the degree of cotyledonary node swelling of 7- to 24-day- 

 old seedlings as a resistance indicator have produced 

 variable results and are not recommended. More reliable 

 evaluation techniques delay inoculation until seedlings are 

 about 2 weeks old with resistance ratings made after 16 

 weeks (1 7). 



Evaluation tests can be conducted either in a greenhouse or 

 growth chamber. A 20°C temperature and 12- to 16-h photo- 

 period are recommended. Seeds of test cultivars are germi- 

 nated on filter paper. When seedling radicals are approxi- 

 mately 5 mm long, the seedlings are planted in rows in 

 metal flats filled with steamed or fumigated soil. A fine or 

 very fine sandy loam soil is preferred. Soil mixes containing 

 high levels of peat moss should be avoided. Seedlings are 

 spaced no closer than 2.5 cm apart in rows, with rows no 

 closer than 3 cm. Seedlings then are covered with about 



30 



