Evaluating Sugarbeet Seedlings for
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Powdery mildew development on the foliage was e v a l u a t e d by v i s u a l l y a s s i g n i n g a r a t i n g of 1 to 5 to each row based on the extent of mycelial growth and sporulation. A r a t i n g of 1 indicated very s p a r s e mycelial growth and no evidence of s p o r u - l a t i o n , whereas a r a t i n g of 5 indicated dense mycelial growth and abundant sporulation.
Cotyledon Method
Plants were grown in the greenhouse for 3 weeks until the cotyledons were fully expanded and the first true leaf was beginning to emerge. A section 1 cm long was cut from a cotyledon of each of 12 p l a n t s per line tested. Cotyledon sec- tions were placed underside up in a randomized complete block p a t t e r n on a sheet of moist filter p a p e r . The sheet of filter paper was positioned at the bottom of an inoculaton chamber consisting of a plywood column 1.2 m high and 0.5 m s q u a r e . Inoculation was accomplished by shaking infected leaves at the top of the chamber so conidia settled uniformly on the surface of the cotyledon sections. Conidia were blown off of infected leaves 2 days before they were used for inoculation.
Therefore, most conidia used as inoculum were less than 48 hr old. Inoculum dosage was measured by placing a g a r strips with the cotyledon sections and counting conidia on the agar surface.
Dosage was routinely adjusted to 8 to 10 conidia per mm of surface.
Each inoculated section was floated on 1 ml of water con- t a i n i n g benzimidalole (Baker Chemical Co., P h i l l i p s b u r g , N.J. 08865) in p l a s t i c 96-cup disposable t r a y s . The t r a y s were enclosed in zip-close polyethylene b a g s to prevent evaporation and incubated for 6 days in a growth chamber. The growth chamber was operated with 10-hr days at 22 C, and 14-hr n i g h t s at 18 C. The t r a y s of cotyledon sections were placed 20 cm below 15-w fluorescent l i g h t s (3,063 l u x ) .
Fungus development on cotyledon surfaces was e v a l u a t e d by examining each section with X 30 magnification. A r a t i n g of 1 to 5 based on extent of mycelial growth and abundance of sporulation as in field evaluations was assigned each section.
Leaf-Disk Method
Seedlings were grown in the greenhouse for 5 weeks or
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u n t i l t h e i r first t r u e leaves were fully e x p a n d e d . A disk 1cm in diameter was cut from n e a r the b a s e of the b l a d e of the first or second t r u e leaf of each p l a n t to be t e s t e d . The d i s k s were cut with a No. 5 cork b o r e r . Disks from 12 p l a n t s were used to e v a l u a t e each s u g a r b e e t l i n e . The d i s k s were placed u n d e r - side up on moist filter paper in a randomized complete block p a t t e r n and inoculated using the same procedure described above for cotyledon sections. Incubation of inoculated d i s k s and e v a l u a t i o n s were also as for cotyledon s e c t i o n s .
Whole-seedling Method
Seedlings were grown in the greenhouse for 5 weeks before i n o c u l a t i o n . The seedlings were randomly a r r a n g e d in a w a l k - in growth chamber with doors t h a t could be closed to reduce a i r c u r r e n t s . Each e n t i r e seedling was inoculated by holding i n - fected p l a n s with a b u n d a n t s p o r u l a t i o n about 60 cm above the seedlings and then s h a k i n g them to dislodge the c o n i d i a . This procedure was r e p e a t e d two or three times to i n s u r e a uniform d i s t r i b u t i o n of c o n i d i a .
The inoculated seedlings remained in the growth chamber for 7 to 8 d a y s u n t i l disease symptoms developed. The growth chamber was operated on a 16-hr d a y (fluorescent l i g h t at 4,500 lux) at 25 C, and 8-hr night at 20 C. Reaction to powdery mildew was e v a l u a t e d by a s s i g n i n g a d i s e a s e r a t i n g of 1 to 5 as described for the other methods.
RESULTS AND DISCUSSION
Significant differences occurred in powdery mildew d i s e a s e r a t i n g s a s s i g n e d to 20 s u g a r b e e t c u l t i v a r s and breeding lines when exposed to n a t u r a l infection in the field (Table 1 ) . Field r a t i n g s were similar for e n t r i e s over 3 y e a r s . Commerical c u l t i - v a r s D2, AH12, HH22, and UI8 were s u s c e p t i b l e . Resistant b r e e d i n g lines such as L37, FC504, ELA0, and L53 h a v e d i v e r s e genetic b a c k - g r o u n d s and may be excellent sources for d e v e l o p - ing c u l t i v a r s r e s i s t a n t to powdery mildew.
The powdery mildew r a t i n g s for the three seedling a s s a y methods a r e l i s t e d in Table 1. Ratings differed s i g n i f i c a n t l y with the cotyledon and l e a f - d i s k methods However, these meth- ods do not w a r r a n t further c o n s i d e r a t i o n because the r a n g e of
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disease reactions w a s narrow and the correlations with field ratings were low. It w a s observed during this study that injury influenced powdery mildew development, and this m a y explain
the lack of correlation between cotyledon and leaf-disk r a t i n g s and field r a t i n g s . The g r e a t e s t injury effect would occur with l e a f - d i s k s , and t h i s method gave the poorest correlation with field e v a l u a t i o n s .
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The whole-seedling method g a v e the highest c o r r e l a t i o n with field r e s u l t s (r = . 6 5 ) . If we exclude lines L53 and 53100- 0 4 , which had higher seedling t h a n field r a t i n g s , and line
56 which had a lower seedling t h a n field r a t i n g , the c o r r e l a t i o n of whole-seedling and field r a t i n g s is r = .78. All nine of the lines t h a t received field r a t i n g s over 3.0 were classified 3.0 or higher in the whole-seedling t e s t . All but three of the 11 lines t h a t received 3.0 or lower field r a t i n g s were classified below 3.0 in the whole-seedling test (Table 1 ) . These r e s u l t s indicate t h a t the whole-seedling method of e v a l u a t i n g for powdery mildew r e s i s t a n c e could effectively identify susceptible lines but would occasionally fail to identify a r e s i s t a n t l i n e . It should be noted t h a t a growth chamber was used for inoculation and d i s e a s e development because t h i s i n v e s t i g a t i o n was not compatible with non-disease work being done in the g r e e n h o u s e . Other t e s t i n g h a s shown, however, t h a t seedling e v a l u a t i o n can be done in the greenhouse, p a r t i c u l a r l y if s h a d i n g is provided d u r i n g periods of intense s u n l i g h t .
This seedling method r e q u i r e s only 6 weeks to complete and can be done anytime d u r i n g the y e a r . Resistant selections a r e not destroyed in the e v a l u a t i o n process but can be recovered for r e p r o d u c t i o n . This method should f a c i l i t a t e the development of r e s i s t a n t c u l t i v a r s and the study of how r e s i s t a n c e is i n - h e r i t e d .
LITERATURE CITED
(1) Kontaxis, D. G. , H. Meister, and R. K. Sharma. 1974.
Powdery mildew epiphytotic on s u g a r b e e t s . Plant Dis.
Rep. 58:904 - 905.
(2) Kontaxis, D. G. 1976. Chemical control of powdery mildew on s u g a r b e e t s . Calif. Agr. 30:13 - 14.
(3) Ruppel, E. G., F. J. Hills, and D. L. Mumford. 1975.
Epidemiological o b s e r v a t i o n s on the s u g a r b e e t powdery mildew epiphytotic in western U.S.A. in 1974. P l a n t Dis. Rep. 59:283 - 286.
(4) Skoyen, I. O. , R. J. Lewellen, and J. S. McFarlane.
1975- Effect of powdery mildew on s u g a r b e e t p r o d u c - tion in the Salinas Valley of C a l i f o r n i a . P l a n t Dis.
Rep. 59:506 - 510.