KERNEL RED STREAK 0N INBRED AND HYBRiD CORN (ZEA MAYS L.) iNFECTED WiTH WHEAT CURL MITES (ACERIA TULIPAE Kt) Thesis for the Degree of M. S. MECHIGAN STATE UNiVERSITY HABIBOLLAH FAKHRAI 1968 'TT‘EEEflEi L I B R A R Y Michigan State Umvcrsity if B";;NG av - "DAG & SENS' 800K BINDERY mr; ['“pAR 'vunr KERNEL RED STREAK ON INBRED AND HYBRID CORN (ZEA MAYS L.) INFECTED WITH WHEAT CURL MITES (ACERIA TULIPAE K.) By Habibollah Fakhrai A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Crop Science 1968 To my father, Mr. Mahmud Fakhrai, and my mother, Mrs. Gohar Fakhrai (Ghotb), for their sincere and dedicated support all through my life. ACKNOWLEDGMENTS The author wishes to express his sincere appreciation to Dr. E. C. Rossman, for his guidance in the study and in the prepa- ration of this manuscript. Thanks are especially due to Dr. C. E. Cress, for his critical appraisal and help in the analyses of the data, Drs. J. Bath and E. Saari, for their help in procedures, and to Drs. C. M. Harrison and H. Slatis, for their critical review of the manuscript. Some financial support from DeKalb Agricultural Association, DeKalb, Illinois, is acknowledged. Thanks are also due to my wife, Farideh, for help and patience throughout the course of this study. iii TABLE OF CONTENTS IJSTCHPTABLES INTRODUCTKflJ REVIEW OF LITERATURE NUYPERIALSZUEDIWETTKMDS RESULTS AND DISCUSSION Mite Counts and KRS Scores, September 30- October6 Yield, % KRS, KRS Color Intensity Score, and KRS Index, October 15 . EHNMRDXRY REFERENCES iv Page 10 14 14 22 39 42 Table LIST OF TABLES Analyses of variance for Kernel Red Streak scores and mite counts sampled September 30- October 6, 1967 . Mean KRS scores for 13 corn inbreds and 11 hybrids with three treatments sampled September 30-October 6, 1967 Mean mite counts of 3 drop samples for 13 inbreds and 11 hybrids with three treat- ments sampled September 30-October 6, 1967 Analyses of variance for percent KRS, color intensity scores, and KRS index sampled October 15, 1967 Mean percent KRS for 13 inbreds and 11 hybrids with three treatments sampled October 15 Mean KRS color intensity scores (0-8) for 13 inbreds and 11 hybrids with three treat- ments sampled October 15 . . Mean KRS index for 13 inbreds and 11 hybrids with three treatments sampled October 15 Correlations between mite counts, percent KRS, KRS color intensity scores, and KRS index Page 15 16 17 23 26 27 28 31 Table 10. 11. Page Classification of 24 corn varieties into KRS susceptible, intermediate, and resistant groups based on Tukey' 5 method applied totheKRSindex.................33 Analysis of variance for yield sampled October15,1967................36 Mean yields bushel per acre for 13 inbreds and 11 hybrids with three treatments, October 15. Plant population = 25, 900 peracre....................37 vi INT RODU C TION Kernel red streak (KRS) was first noticed on corn in southern Michigan, northern Ohio, and northern Indiana in 1963. The infected area increased during 1964 in the three states, and into Ontario, Canada. Extensive KRS developed in Illinois in 1966. Pennsylvania also reported KRS on corn in 1966. KRS has been reported in Bulgaria, France, Romania, and Yugoslavia (11). Longitudinal streaks of red color in the pericarp are the main visible symptoms of Kernel Red Streak. The red color does not extend into the endOSperm tissue. The streaks usually spread upward from the tip end of the kernel. With heavy infection, the streaks coalesce and a major portion of the kernel becomes solid red to purple in color. According to federal grain standards, corn may be graded "mixed" and discounted when 5% or more of the kernels in a sample are red colored. A kernel is not counted as red unless 50% or more of the kernel is red colored. Discounts for KRS have been applied rather sporadically over the area and are not being applied cur- rently. There have been no reports of adverse effects on feeding quality of KRS infected grain (21). The causal agent remained obscure until a virus, Ohio 3A strain of Wheat Streak Mosaic Viris (WSMV), was isolated from KRS infected plants by Williams et_al. (26) in 1965. Two years later, Nault 31.11. (11) showed that the insect vector of WSMV, the microscopic wheat curl mite (Aceria tulipae Keifer), was the pri- mary causal agent and not WSMV per se. KRS developed on the kernal pericarp as a result of feeding by virus-free mites as well as by viruliferous mites. They suggested that a phytotoxin, possibly in the saliva, produced the red streaking. Plants infected with WSMV appeared to be better hosts for the mite, but plants inoculated with WSMV alone did not develop KRS. WSMV and its vector have been present for many years in the Great Plains wheat area where the virus is a major disease of wheat. KRS has not been found on corn in this area. WSMV has been a relatively minor disease of wheat in the Michigan, Ohio, and Indiana area where KRS suddenly appeared on corn in 1963. Explanations are not clear why KRS does not develop on corn in the Great Plains and does develop in certain areas of the Midwest. Possibly a different strain, a "corn strain" of wheat curl mite, has evolved in the Midwest. Rossman (16) reported that a higher degree of KRS developed when yields were low and moisture stress existed during the season. KRS was also present on grain from high yielding fields but usually less intense. Does KRS affect yield? Attempts to produce comparable plots with and without KRS under field conditions had been unsuccess- ful until the causal agent was clearly determined in 1966. The appli- cation of systemic insecticides for mite control appeared to be a possibility for a set of KRS-free plots to compare with plots artifi- cially infested with mites to produce KRS. Objectives were to study the effects of three treatments (virus free mites, WSMV infected mites, and control with systemic insecticides) on development of KRS and on yield. Twenty-four genotypes (13 inbreds and 11 hybrids including red and white cob materials previously rated relatively susceptible and resistant to either WSMV or KRS) were used in this study. REV IEW OF LITERATURE Williams Stil' (26) obtained a virus, Ohio BA, from KRS infected plants and suspected that it was the causal agent. The mosaic pattern on the leaves was described as a series of dots and dashes. This virus has since been shown (10, 11) to be a strain of WSMV. Paliwal 111. (14) in 1966 reported that there was no indi- cation that WSMV produced KRS. They were unable to isolate WSMV from locations Where KRS occurred in Ontario, Canada. Nault gt_a_1. (11) found that the WSMV insect vector Aceria tulipae K. (wheat curl mite) and not WSMV was the primary cause of KRS. They found a high correlation between the presence of mites and KRS. Inoculation with virus free mites as well as with virulif— erous mites produced KRS. They concluded that presence of the virus was not essential for development of KRS, but might make the plant more attractive to the mite. Since wheat curl mites are known to have toxigenic effects on several hosts, it seemed possible that KRS might be due to phytotoxin secreted by the feeding mite. Everly (2) also found that mite infection was associated with KRS but some samples with KRS had no mites and some samples with no KRS had mites. WSMV has rarely occurred with any significant effect or with any degree of regularity on corn. Its potential danger has been noted (4) but the limited population of susceptible corn could not itself cause an epidemic of WSMV on corn. McKinney (9) inoculated 8 varieties of dent corn and 13 varieties of sweet corn with Wheat Streak Mosaic Virus in the greenhouse. The percentage of plants with mosaic symptoms ranged from 0—68 percent. Plants with severe mosaic were stunted but none were killed. Five varieties (1 dent and 4 sweet) developed no mosaic. No data were reported for grain appearance. W. B. Allington (University of Nebraska, personal com- munication) observed that the pollinator parent of a commercial corn hybrid in the North Platte area of Nebraska was highly suscep- tible to WSMV. In several seed fields the pollinator parent was dead soon after pollination while the seed parent in the same field was unaffected. No KRS was ever observed on corn in Nebraska, where WSMV is common on wheat. Other states in the Great Plains wheat area where WSMV has been a problem for many years report no KRS on corn. King and Sill (7) estimated a 20% loss in wheat yield in Kansas due to WSMV. Presence of WSMV in corn and wheat has been found to create greater susceptibility of the host to fungal crown and root rots (22). The susceptible host range of WSMV includes several spe- cies of wheat and oats, some varieties of corn, barley, rye, and wild grasses such as Aegilops cylindrica Host, Bromus japonicus Thumb, and others (3). Sill and Connin (17) reported that WSMV overwinters on wheat. Susceptible summer grasses are distributed widely enough to serve as interim hosts between wheat harvest and planting, and furnish a source of inoculation for each new wheat crop. Slykhuis (19) reported effective control of WSMV and wheat spot mosaic by eliminating immature volunteer wheat before winter wheat was planted. The continuous sequence of wheat necessary to perpetuate the disease was interrupted. Staples and Allington (22) stated that destruction of native grasses was not important or prac- tical for control of WSMV. Slykhuis (18) was the first to report the wheat curl mite (Aceria tulipae K.) as the insect vector for WSMV. The virus could be transmitted by all forms of the insect except the egg. Connin and Staples (1) found that the mite occasionally clings to other insects and escapes detection. Staples and Allington (22) reported that annual host grasses provided a source of the virus and were suitable hosts for the mite. They found that wind was the main method of mite dispersal and some mite migration occurred throughout the year. Volunteer wheat ger- minating after harvest also served as an interim host for the virus and the mite. Orlob (12) reported that most annual grasses were either immune to WSMV or did not support mites but certain grasses such as Setaria viridis served as interim hosts for the virus or vector. In the fall, viruliferous mites from grasses such as Setaria viridis were transported to winter wheat and introduced the virus. He stated that since Aceria tulipae K. from these grasses do not adapt readily to wheat, wheat adapted mites would be needed to spread the virus within the field. Intra-field spread of WSMV from infected source plants was confined to short distances. It appeared that viruliferous mites were blown into most fields without causing exten- sive intra-field spread. McKinney St_a_l_. (10) stated that wheat and susceptible corn varieties could be year-round hosts for both virus and mites through- out the Corn Belt and the eastern Great Plains. Keifer (6) first described Aceria tulipae and gave average measurement of 250 microns in length and 75 microns in width for the adult . Mites thrived best in the greenhouse under high humidity, but flooding for 24 hours or more, was detrimental and eventually lethal (15). Phototropic response of mites was negative. Egg to egg cycle at 75-78 F was found to be 8-10 days (15, 22). Mites reproduced parthenogenetically. At least 12 eggs were produced by each female. Leaves of young’wheat plants infested with mites became rolled, parallel to the veins, from one edge to the other (22). Mites progressed from‘older leaves to new leaves. On corn, mites gain access to the kernels through the silk and tip of the ear (11). L. R. Nault (unpublished) reported that mites appeared on corn early (plants 4-6 inches tall) in the season and continued to increase during the season in 1967 in Ohio. None of 30 different insecticides applied in greenhouses and in the field in the spring gave initial or residual control of mites (5). Leaf rolling and penetration beneath leaf sheaths gave protection to the mites. No control of WSMV was obtained when seeds were treated with Am. Cyanamid 12008, 12009. Kernel Red Streak is most common on yellow dent corn and least common on white corn (11). It has been noted on sweet, pop, and flint corns . Rating of inbred lines of corn by Rossman (16) and others showed that white cob inbreds tended to be relatively free of KRS compared to red cob material. Some streaking was noted on a few white cob inbreds. Likewise, some red cob lines were relatively free of streaking. Rossman (16) found consistent differences in amount and intensity of streaking among commercial hybrids in overstate corn performance trials in Michigan. None were completely free of KRS. All were red cob hybrids. Unpublished tests by Rossman have shown that no infectious principal was carried by seed with KRS. Heavily streaked seed from Michigan was planted in the Florida winter corn breeding- genetics research nursery in 1963 and 1964. No KRS developed any- where in the Florida nursery. KRS occurred more commonly at the ear tips, particularly if husks were loose and kernels exposed, but also appeared randomly on the ear (11). KRS increased in amount and intensity progressively from slightly before denting until heavy killing frost in the fall (16). MATERIALS AND ME THODS Three treatments were applied to 24 corn varieties (13 inbreds and 1 1 hybrids) in a split plot design with four replications on the Michigan State University Crop Science Department Experi- mental Farm near East Lansing in 1967. The three treatments were: T1 Control plots were treated with systemic insecticides in an attempt to keep natural infestation of mites at a minimum. Two applications of NIA 10242 (2, 3-dihydro-2—2—dimethyl- 7-benzofuranyl methyl carbamate) were applied to the soil, 14 days and 75 days after planting. Three applications of dimethoate-ZE were applied to silks on August 15, Septem- ber 1, and September 15. Viruliferous mites, reared on Monon wheat plants inoculated with isolate 931 of WSMV by Dr. E. G. Saari (Department of Plant Pathology, Michigan State University) were placed into the ears soon after silking. Virus-free mites were reared on uninoculated wheat plants and transferred to corn ears soon after silking. 10 11 For both treatments (T and T3), wheat leaves were in- 2 Spected for presence of mites and then cut into small sections con- taining about ten mites. The small pieces of leaves were inserted into the tip of each ear between the silk and husk. Hybrids were infested with viruliferous mites and with virus-free mites on August 2 and 3; inbreds were infested August 14 and 15. The corn was hand planted May 13. Excess seed was planted and the plants were thinned later to a population of about 25, 900 per acre. Plots were one row 14 feet long with 25 plants. All plots received 300 pounds 10-20-20 fertilizer in the row and 120 pounds nitrogen (anhydrous ammonia) sidedressed in mid-June for a total of 130-60-60 pounds of N-PZOS-KZ0 per acre. No ”effective” rainfall (. 4" or more) occurred from July 20 to August 19 and some moisture stress developed on the plants during this period. The 13 inbreds included seven lines with red cob (M880, W9R (22), M8140, M8142«Rf, W153R, Oh51A, and M8153 Rf) and six with white cob (B8, M8141, M8142 Rf, M857, SD10, and M8153 Rf). Two inbreds, M8142 Rf and M8153Rf, were represented by both a red and a white cob segregate obtained from a backcross program that converted these two lines to pollen restorers. The 11 hybrids consisted of two double cross hybrids, Michigan 270 and Michigan 400, and nine single cross hybrids. M8141 x B8 and 12 M81334 x Oh43 involve only white cob inbreds in their pedigree. M8140 x B8, M8142 x B8, M8142 x M8141 and M8140 x M8141 are crosses of red cob x White cob inbreds. M8142 x M8140 and Michigan 402-2X (W64A x WF9) (M892 x MS93) are red cob x red cob crosses. DeKalb XL15 is closed pedigree single cross with red cob. Michigan 270 (M81334 x B8) (W10 x M8206) is a cross of a white cob single cross with a red cob single cross. Michigan 400 (W64A x Oh43) (W10 x M8206) has one white cob inbred, Oh43, in its pedigree. One ear from each plot was harvested September 30- October 6 for mite count and KRS rating. The procedure developed by Dr. James Bath, Department of Entomology, Michigan State University, was used for mite counts. The ear'with husk attached was cut, sprayed with chloroform in a jar and washed with 190 proof grain alcohol. Three U. 8. standard sieves, Numbers 10, 70 and 325 (with openings of 2 mm, 125 microns and 40 microns, respec- tively), were used to strain off the mites in the alcohol solution. Mites collected inside of the No. 325 screen were washed off with alcohol into a vial with a volume of 1640 drops. A three drop sample on a concave slide was taken for the mite count using a 40 times magnitude binocular. 13 The degree of color intensity (scored as 0, 0. 5, 1. 5, 2. 0, 2. 5 or 3. 0 with 0 being no color and 3, 0 representing heaviest coloration) was determined for each one ear per plot sample. Two types of correlations of mite counts with KRS ratings were calculated: 1. simple correlation over all treatments and varieties, and 2. within correlation eliminating treatment within variety effects. The final harvest was taken on October 15 when 10 plants per plot were harvested for yield determination and KRS rating. Plot weights were converted to bushels per acre at 15. 5% moisture. The 10 ears were shelled after drying. A bulk sample of 100 kernels was counted to obtain the percentage of kernels showing KRS. The kernels showing KRS in the sample were rated for inten- sity using scores of 0 (no color) to 8 (heavily colored). An index (Z) for KRS was calculated for each plot using the formula: 2 X -Y Z"10*‘3~'10< 100 )+1 where X is the color intensity score and Y = percent kernels with KRS. This index is somewhat arbitrary, but is used in an effort to obtain a measure of total severity of KRS infestation. RESULTS AND DISCUSSION Mite Counts and KRS Scores, September 30-October 6 One ear per plot was harvested during September 30- October 6, 1967, for wheat curl mite counts (Table 3) and scored (0-3) for KRS (Table 2). Analyses of variance are presented in Table 1. Screened mites from each ear were washed into a vial con- taining about 1640 drops. A three drop sample (one 547th) was taken from the vial for the mite count. Multiplying the count by 547 gives an estimate of the mite population per ear. The effect of treatment and varieties and their interaction were highly significant for both mite counts and KRS scores (Table 1). The control treatment (T1 = systemic insecticides) had significantly lower mite populations (averaging 2. 32) and significantly lower KRS scores (averaging . 89) than ears of either of the other two treatments which were manually infested with mites. Ears infested with viru- liferous mites (T2) had an average mite count of 14. 73 and an average KRS score of 1. 70. Infestation with virus-free (T3) averaged 16. 14 on mite counts and 1. 73 on KRS scores. 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Treatmentb Comparisons Variety Cob colora '1‘l vs T1 T2 T3 'r a T T2 V5 T3 2 3 Inbreds: M880 Rc 0.00 1.75 1.00 ** 4‘ W9R(22) Rc 1.13 3. 00 2. 75 ** ns BB Wc 0.38 0.63 0.38 ns ns M8141 Wc 0.00 0.00 0.00 ns ns M8140 Rc 2. 13 2.88 2.88 * ns M8142 Rf Rc 1.13 2.75 2. 50 *‘4 ns M8142 Rf We 0. 13 0.13 0.13 ns ns W153R Rc 0.75 2.00 2.50 ** ns 0115111 Re 1. 75 2. 88 2. 75 M as M857 We 1.13 1.13 1.13 ns ns SDIO Wc 2.00 2.00 1.88 ns ns M8153 Rf We 0.13 0. 83 0. 63 ns ns M8153 Rf Rc 0.63 2.00 2.38 ** ns Mean of 13 inbreds: 0.87 1.70 1.61 * ns Mean of 8 white cob inbreds: 0. 83 0. 80 0. 69 ns ns Mean of 7 red cob inbreds: 1. 08 2. 47 2. 39 ** ns Hybrids: M8140 x BB Rc = Be x We 1.25 2 13 2.13 ** ns M8141): BB Wc = We x We 0.25 0 75 O. 88 ns ns M8142 x BB Rc = Be x We 0 50 1 50 1.25 ** ns M8141): M8140 Rc = We it Re 1 00 1 50 2. 13 ** ns M8142 x M8140 Re a BC it Re 1 50 2 88 2. 88 ** ns M8142 x M8141 Rc = Re x We 0 50 1 75 1.38 ** ns M81334 x Oh43 We = We x.Wc 0 75 1 00 1.25 ns ns Mich. 270 (MS1334 x BB) x (W10 3: M8206) Rc = (We x We) at (R0 it Re) 1 00 1 75 2. 50 ** * Mich. 402-2X (W64A x WF9) x (M892 x M893) Rc = (Re it Be) it (Re it Re) 1 13 1. 63 2. 13 * ns Mich. 400 (“IBM 3: Oh43) x (W10 2: M8206) Re a (Re x We) it (Re it Re) 0. 63 1. 83 1. 50 ** ns DeKalb XL15 closed pedigree Re 1. 83 2 50 2. 75 ** ns Mean of 11 hybrids: 0. 92 1. 73 1. 90 ** ns Mean of 2 white cob hybrids: 0 50 0. 88 1. 07 ns ns Mean of 9 red cob hybrids: 1. 02 1 92 2. 10 ** ns Mean of all 24 varieties: ‘ 0. 89 1 70 1. 73 “I ns Mean of- 8 white cob varieties: 0 80 0 82 0. 79 ns ns Mean of 18 red cob varieties: 1 ’50 2 18 2.24 * ns Least Significant Difference: 0. 80 0 64 0. 82 alite :- red cob; We a white cob. le : control with systemic insecticide; T T = manual infestation of ear with virus-free mite 3 ** I significant at 0. 01 level of probability. * a significant at 0. 05 level of probability. ns - not significant. 3 s manual infestation of ear with viruliferous mites; 17 Table 3. -- Moan mite counts of 3 drop samples i'm- 13 inbreds and 11 hybrids with three treatments sampled September 30-October 6, 1067, Treatmentb Comparisons Variety Cob colnra T1 VS T1 T2 T3 '1" a T T2 VS T3 2 3 lnbreds: M880 RC 3. 00 18.50 13.50 ”"3 ns W9R(22) Rc 1.50 31.00 25.25 ** ns BB We 2.00 4.00 5.25 ns ns M8141 Wc 2.00 12.75 20.25 W ns M8140 RC 4.25 8 50 24 50 ‘* * M8142 Rf Rc 1.50 18.75 20.50 “A ns M8142 Rf We 2.25 15.25 7.75 ns W153R He 2.75 18.25 11.75 ** ns Oh51A Rc 6.25 19.75 23.75 ”J" ns M857 Wc 1.75 23.25 19.50 ’1“3‘ ns SD10 We 2.00 5 25 15 25 * M8153 Rf We 1.25 12.25 6. 75 =1: ns 11118153 Rf RC 1.50 10.25 16.50 =1”? ns Mean of 13 inbreds: 2.46 15.21 16.20 *5“- ns Mean of 6 white cob inbreds: 1.86 12.13 12. 46 ** ns Mean of 7 red cob inbreds: 2. 96 17. 86 19. 54 ** ns Hybrids: M8140 x B8 RC = Re x We 0.50 11.50 17.00 ** ns M8141): 88 We = We x We 1. 25 3. 50 10. 50 ns ns M8142 x BB Rc = Re it Re 2.25 9.25 11.00 ’5‘ ns M8141 x M8140 Rc = We it Re 1. 50 11.25 13.50 ** ns M8142 x M8140 Rc = Re x Rc 3. 75 26. 50 25. 50 ** ns M8142 x M8141 Rc = Rc x Wc 2. 75 16. 00 15.75 ** ns M81334 3: Oh43 We = We at We 1. 00 18. 75 16. 50 ** ns Mich. 270 (M81334 x BB) x (W10 x M8206) RC = (We x Wc) x (Rc x Rc) 1 75 9.00 14.75 ** ns Mich. 402-2X (W64A x WF9) x (M892 x M893) Rc = (Re x Re) x (Rc x RC) 3 50 9 25 20. 75 M * Mich. 400 (W64A x Oh43) x (W10 x M8206) Rc = (Re x We) x (RC x Rc) 3. 00 19. 50 16. 75 ** ns DeKalb XL15 closed pedigree Rc 2. 50 21. 25 14. 75 ** ns Mean of 11 hybrids: 2.16 14. 16 16.00 M . ns Mean of 2 white cob hybrids: 2. 12 11.13 13.50 We ns Mean of 9 red cob hybrids: 2. 38 14. 83 16, 67 a:==:< ns Mean of all 24 varieties: 2. 32 14. 73 16. 14 M‘ n8 Mean of 8 white cob varieties: 1.93 11. 88 12. 72 W ns Mean of 16 red cob varieties: 2. 63 16. 16 17. 93 ** ns Least Significant Difference: 2 60 12.00 8 44 Rc = red cob; We = white cob. T1 = control With systemic insectiCide; T = manual infestation of ear With viruliferous mites; T3 = manual infestation of ear with virus-free mites. ** = significant at 0. 01 level of probability. * significant at 0. 05 level of probability. ns not significant. II II Multiply mite counts by 547 for the estimated mite population per ear. 18 between ears infested with viruliferous mites and those infested with virus-free mites (T vs T3) were not significant for either mite 2 counts or KRS scores. Ears that were manually infested with mites (T and T3) averaged 6 to 7 times more mites and about twice as 2 high on KRS scores. Effectiveness of the systemic insecticides (NIA 10242 = 2, 3, dihydro-2-2-dimethyl-7-benzofuranyl methyl carbamate applied to the soil and dimethoate 2-E applied to the silk) for wheat curl mite cannot be judged here since there was no treatment where insecticides were not used. However, insecticides did not provide complete control of mites since control ears averaged about 1369 mites (2. 32 x 547) per ear. The control treatments (T1) ranged from a low of 0. 5 mites per sample for M8140 x B8 to a high of 6. 25 for Oh51A. When manually infested, M8140 x B8 had 11, 50 (T2) and 17. 00 (T3) mites per sample, representing a 23- and 34—fold increase over the con- trol. For all varieties there were 6. 6 and 7. 5 times more mites for the two manually infested treatments (T2 and T3) than for the control treatment. Manual infestation of ears with viruliferous mites did not increase the KRS scores compared with ears infested with virus-free mites. There were only three instances (M8140, SD10 and hybrid 19 Michigan 402-2X) of a significant difference between these two treat- ments (T2 ence for KRS (inbred M880 and hybrid Michigan 270). vs T3) in mite counts and two cases of significant differ- No visual symptoms of Wheat Streak Mosaic Virus (WSMV) were detected on any of the plots during the growing season. The viruliferous mites (T2) were reared on Monon wheat plants inoculated with WSMV, and, as such, were assumed to be carrying the virus. Virus transmission tests (corn to Monon wheat) were made for all varieties in T2. None were positive, indicating that the mites reared on Monon wheat infected with WSMV did not transmit the virus to the corn. Inbred Oh51A had been rated highly susceptible to WSMV according to unpublished tests conducted by L. E. Williams in Ohio. Virus transmission tests (corn to Monon wheat) for Oh51A were negative. There was a natural population of mites in the area as evidenced by the mite counts obtained from the control treatments. Wheat research plots were adjacent to the corn plots and could have been the source of the natural infestation. Some of the mites present on the manually infested plots (T2 and T3) were probably from the natural population in the area. It is not known whether the natural population of mites were viruliferous or virus-free. Nault et al. 20 (11) found that mites from only two of 68 cars collected from fields in Ohio transmitted WSMV. Infestation with viruliferous mites did not increase KRS over that obtained with virus-free mites. Since there was no evi- dence of WSMV from visual symptoms on corn or from corn to Monon wheat transmission tests for T2 and T3, it appears that the development of KRS was largely independent of the WSMV. Con- trolled experiments by Nault et a1. (11) have demonstrated that KRS developed on ears infested with virus-free mites. Paliwal et al. (14) found that KRS developed on plants that had no WSMV and also on plants that had WSMV, based on transmission tests. They also isolated WSMV from plants that had no KRS and concluded that WSMV was not the cause of KRS. The within correlation (removing treatment within variety effects) of mite counts with KRS scores was . 72 which was highly significant (Table 8). This relationship indicates that the higher KRS scores were generally obtained on ears possessing higher mite populations. Nault 3E. (11) obtained significant correlations ranging from . 47 to .67 for mite counts with KRS incidence. The correlation of mite counts with KRS scores was . 64, very highly significant, for 16 red cob varieties but was not significant, . 16, for 8 white cob varieties. White cob varieties showed relatively 21 little KRS even though there were appreciable mite populations on them. Red cob inbreds and hybrids averaged higher in mite counts and in KRS scores than white cob inbreds and hybrids for all three treatments (Tables 2 and 3). KRS scores averaged two to three times higher for red cob inbreds and hybrid groups than for white cob groups. Mite counts averaged only 1. 38 times higher for red cob than for white cob, while KRS scores averaged 2. 47 times for red cob varieties. White cob inbred M8141 showed no KRS in either treatment yet it had about average mite counts. The white cob version of M8142 Rf had the next lowest KRS score (. 13) in all 3 treatments and also had mite counts as high as several inbreds and hybrids (both white and red cob) with relatively high KRS scores. These two exceptions to the . 72 correlation of mite counts with KRS scores indicate that a low KRS score was not due to absence of mites. This is illustrated further by the correlation between mite counts and KRS scores for the eight white cob inbreds and hybrids, . 16 (Table 8), which is not significant. Nault eta—l. (11) postulated that a phyto- toxin, probably a salivary component, may be secreted by the mite and result in the red streaking on the pericarp. The KRS ”resistant" white cob lines may lack a pigment precursor in the pericarp that, 22 when present in KRS ”susceptible" varieties, reacts with the excreted salivary phytotoxin of the mite to produce red streaking on the peri- carp. The average mite counts and the average KRS scores for the 13 inbreds were similar to the average for 11 hybrids. There was no consistent indication that the inbreds as a group were any more or less attractive to mites than the hybrids. Yield, % KRS, KRS Color Intensity Score, and KRS Index, October 15 Ten plants in each plot were harvested for yield on October 15. No mite counts were made at this final harvest. The percentage of kernels with KRS and the intensity of the color (scores: 0 = no KRS color to 8 = pericarp almost completely red colored) were determined using a 100 kernel sample of the shelled grain from each plot. A KRS index was calculated using the formula: 2 Index KRS = Z = log (gTfiS—K— + 1) where X is the KRS color intensity score and Y is the percent KRS. Analyses of variance for percent KRS (means in Table 5), color intensity scores (means in Table 6), and KRS index (means in Table 7) are presented in Table 4. There were highly significant differences due to treatments and varieties for all three measurements 23 5232812 so 29,2 :5. a... Eeoflcmsmm no.0 ow.w ed «.3 ~42 ~23va pom mconam . . . . . . . . . 3258305 mwm m. om o 3 m mm n v N fl «2 m m m >2. H mmmmm me x 53.2; www.mm A: .m 3 .me made Rom macs Rom oat: 93:3 mm .3523» «6.0 mad «1o m.~ we: Rpwmv m