sidered resistant. The level of fusarium wilt resistance 

 among cultivars within a test can be expressed as the per- 

 centage of resistant plants or as ASI. Disease ratings can 

 be compared among cultivars by relating each cultivar to 

 the standard resistant check cultivar. The same procedure 

 can be used with acceptable accuracy in the greenhouse by 

 transplanting inoculated plants into pots or benches. 



Phytophthora Root Rot Resistance 



F. I. Frosheiser and D. K. Barnes 

 University of Minnesota, St. Paul 



Field Methods 



Select a level area with relatively heavy soil. If the soil is not 

 naturally infested with Phytophthora megasperma Drechs. 

 f. sp. medicaginis, infested soil may be spread over the area 

 and incorporated into the upper 15 cm. Infested soil can be 

 from naturally infested fields or from greenhouse experi- 

 ments. 



Seed is planted in early May at about 75 viable seeds per 

 1.6 m of row in rows about 30 cm apart. A minimum of three 

 replications per entry is suggested. A stand count is taken 

 approximately 2 weeks after planting while the seedlings 

 are in the unifoliolate leaf stage. After the seedlings are 

 about 4 weeks old, rainfall must be supplemented with irri- 

 gation to keep the surface soil continuously at or near satu- 

 ration for 2 to 3 weeks. After the first 2- to 3-week wet period, 

 the soil is allowed to dry. This allows time to take notes, 

 clip, and cultivate. If the disease is severe, the plants may 

 be evaluated at this time; however, one, and preferably two, 

 additional 3-week wet periods reduce the frequency of es- 

 capes. These additional periods provide a more accurate 

 evaluation of the degree of root rot on each plant because 

 the roots are larger. Plants are dug and scored during the 

 first 2 weeks of September. 



When plants are dug, as much of the root system as possi- 

 ble should be retained. At least 30 cm of taproot are needed. 

 All the plants from a row are dug and tied in a bundle. The 

 roots are soaked in a tub of water and then are washed with 

 water under pressure. Plants are individually classified for 

 degree of root rot. We prefer to do all classification under a 

 uniform light source in the greenhouse rather than in the 

 field. 



A 1 to 6 classification scale described by Frosheiser and 

 Barnes (24) is used: 1 = roots clean with no lesions and 

 many small rootlets present on the taproot; 2 = only very 

 small superficial lesions (2 mm) present on taproot, taproots 

 usually lack numerous branch roots, and most lesions 

 occur at site where branch root had started growth; 3 = one 

 or more large lesions on taproot, but none girdling the tap- 

 root, the tips of one or more larger branch roots rotted off; 



4 = extensive root lesions with the taproots usually rotted 

 off 10 cm or more below the crown; 5 = taproot almost 

 completely destroyed, but plant alive; and 6 = plants dead 

 (calculated as loss in stand count from 2 weeks after seed- 

 ing to mid-Sept.). Plants rated 1 and 2 are considered resist- 

 ant. All tests should contain a resistant check and a suscep- 

 tible check. 



Level of resistance among cultivars within a test can be 

 expressed as percentage of resistant plants or as ASI. Per- 

 centage of resistant plants can be used to compare culti- 

 vars in different tests. This procedure was described previ- 

 ously in the bacterial wilt evaluation procedures and con- 

 sists of expressing the data from each test as the percent- 

 age of the resistant check cultivar, 'Agate'. The data are 

 adjusted to 'Agate's longtime average of 43 percent resist- 

 ant plants. Most cultivars grown in the United States have 

 been tested at the University of Minnesota. 



Greenhouse Method s 



Greenhouse tests are conducted in 20-cm-deep, watertight 

 containers filled with fine, steamed sand (24). The sand is 

 infested initially by using laboratory cultures. The fungus is 

 grown on V-8 juice agar (57) (200 ml V-8 juice, 20 g agar, 1 g 

 CaC0 3 in 800 ml water) in 9-cm petri dishes. Two-week-old 

 cultures are blended in water and then added to the sand in 

 one petri dish to 500 cm 2 surface area. The inoculum is 

 mixed with the sand. 



Seed is planted directly into the sand, and the seedlings are 

 watered sparingly until they are well established (about 

 4 weeks). The drain holes in the tanks are plugged, and 

 enough water is added daily to raise the water level to the 

 sand surface. Sand temperatures of 20° to 24°C are opti- 

 mum for Phytophthora root rot. After about 4 weeks of con- 

 tinual flooding, the plants can be evaluated for root rot as 

 described in field evaluations. Resistant and susceptible 

 check cultivars should be included in each test for 

 comparisons. 



Except in unusual cases, no additional inoculum needs to 

 be added to the sand for repeated testing. Field and green- 

 house evaluations were correlated (r = 0.99 and 0.95) in 

 two tests (24). We think field tests give a more accurate eval- 

 uation of cultivars, however, because the roots are larger 

 and easier to score and more plants can be observed. Never- 

 theless, greenhouse procedures are useful for screening 

 and are being used successfully by a number of plant breed- 

 ers (30). 



25 



