REPORT TO THE AGRICULTURAL RESEARCH FOUNDATION FOR THE OREGON PROCESSED VEGETABLE COMMISSION December 21 Project Title: Management of Fusarium diseases of sweet corn in the PNW: microflora influence on disease and development of Fusarium-free seed Principal Investigator: Cynthia M. Ocamb, Ext. Specialist & Associate Professor Botany and Plant Pathology, OSU Corvallis Phone: (541)737-42, e-mail: ocambc@science.oregonstate.edu Co-investigator: Nathan Miller, Postdoctoral Research Assistant, BPP, OSU Collaborators: Jim Myers (Dept. of Horticulture, OSU) and Rogers Brand Vegetable Background: Sweet corn growers in the Willamette Valley of Oregon reported declining yields during the early 199s. The decline in yields was associated with leaf firing, where the leaves die prematurely starting at the base of the plant and then progress up the plant. Initially the firing and associated yield loss was thought to be caused by root rot. Recent investigations have shown that crown rot, accompanied by a stalk node rot, appears to explain at least partly, perhaps greatly, the concomitant loss in yield. Our pathogenicity experiments show that Fusarium spp. (F. oxysporum, F. verticillioides, and F. proliferatum) can have a negative impact on sweet corn health and yield but other factors are involved as well when yield declines involve Fusarium diseases; such as characteristics of specific corn genotypes, environmental conditions which stress plants or favor Fusarium, or microbial population structure surrounding and within infection points of the corn plant. Commercial sweet corn seed lots have been found to contain high percentages of seed infested by Fusarium; F. verticillioides and F. proliferatum are commonly found. These two species are reported to cause seed rot, seedling blight, stalk rot, root rot, or ear rot, thus sweet corn seed is usually treated with fungicides which normally prevent seedling rot and dampingoff. However, fungicidal seed treatments may not prevent seedlings from becoming infected by seed-borne Fusarium. Not all strains of specific Fusarium species are equal in virulence and some strains are reported non-pathogenic, especially F. verticillioides strains which some argue to be endophytic. Different species of Fusarium can cohabitate the same corn plant and the interaction among strains on seed and with populations of Fusarium in the soil is not understood. Removal of all Fusarium from seed and planting of these seeds does not necessarily reduce losses from Fusarium diseases if pathogenic populations are present in the soil, but removal of seed-borne Fusarium and subsequent biological seed treatment can lead to reductions in root rot severity and crown rot incidence as well as improving yields. So perhaps a weaker pathogen that is present on seed primes the plant for subsequent infection by more aggressive soilborne strains of Fusarium. Objectives for 21 and Accomplishments: 1. Examine the yield and disease levels of sweet corn plants grown from seeds treated with germicidal light. Treating sweet corn seed with UV light reduces the seed-borne contamination levels of Fusarium verticillioides and tends to reduce subsequent infection levels in plants grown from treated seed. Disease in the crown also tends to be reduced under some certain conditions
when seeds are treated with UV light. Ear yield has been increased in previous experiments for some varieties but this season s results seemed inconclusive for the effects of UV seed treatment on yield. The cool, wet climate during the early part of the 21 season may be the reason yield was not affected. Nontreated seeds of 14 different seed lots were obtained from four seed companies. Samples of each seed lot were treated with UV light for one hour. Nontreated and UV- treated seeds were planted in seven or six blocks in two fields at the OSU botany field lab. One field (Field 1) has been grown in continuous sweet corn for ten years and was initially inoculated with root balls from a field with high levels of root rot. The other field (Field 2) has no recent history of corn but has been cropped to wheat and fallow during the past decade. Plants were sampled twice, 6 and 12 weeks after sowing, for evaluation of root and crown rot as well as Fusarium infection levels. Ear weights were sampled at maturity. There was no significant effect of UV light treatment on seed germination (P = for Field 1, P =.73 for Field 2). There was however, variation throughout the fields in seed germination and this variation had a significant effect on ear yield within the plots. To account for this, germination was included in statistical analysis of ear yields, but not for other variables since germination had little or no effect on other variables measured. The two fields were analyzed separately for all disease and yield variables. Most seed lots were highly contaminated with Fusarium verticillioides and treating them with UV light significantly reduced the incidence of Fusarium though the levels of other fungi are sometimes nearly maintained (top two graphs in Fig. 1). Subsequent infection levels in root, crown, and stalk node tissues in plants grown from these seeds were also generally reduced, but not for all sweet corn varieties examined (Figure 1, 2, 4, and 5) and it is apparent that Fusarium infection levels are higher overall for field 1, our Fusarium crown rot field since 2, compared to the new planting, field #2. Crown rot, measured in grayscale, was overall similar for both UV-treated and Nontreated plants at 6 weeks after planting but by 12 weeks after sowing, crown rot was significantly reduced when seeds of half of the varieties were treated with UV light. Our previous studies have also showed reductions in crown rot resulting from UV seed treatment. The effect of UV light seed treatment on ear yield tended to be insignificant for this season, probably because root rot was more severe this season than normal years, due to the cooler, wetter conditions that prevailed through most of the field study. in field 1 had significantly higher yield, and this variety tends to yield better under high crown rot pressure while another variety, =, 27 seed year, in the new field (field 2) had significantly lower yield. The lack of yield improvement during 21 with UV-treatment is probably due to the unusually cool early season, which could reduce the severity of crown rot (which is modulated by the seed microflora) but increase the severity of root rot. The cooler, wetter conditions favored the population of Fusarium in the soil, allowing the soilborne population to overcome the potential benefits of UV-treatment via increased root rot.
1.2 1.2 Node 1 SCI P < 1 Nontreated UV N P = 588 N P < 1 N PR Crown P = 5 N P < 1 P = 4 N N Figure 1. Field 1 -- Proportion of seed infested with Fusarium verticillioides or any fungi before and after being treated with UV light for one hour and proportion of plants with crown, 1 st stalk node, subcrowninternode, or primary root infested with F. verticillioides 6 weeks after planting. P-values in the upper part of each graph indicate significance level for the difference between UV-treated vs. across all seed lots. Individual seed lot comparisons with significance at P < 5 are indicated with. 1.2 1.2 Proportion with Any Fungi
.7.5.3.1 N Node 2 Crown P < 1 P = 267 N N Node 3 Node 1 P = 2 N P =.99.5.3.1 Figure 2. Field 1 -- Proportion of plants with crown and stalk nodes (1 st through 3 rd above crown) infested with F. verticillioides 12 weeks after planting. P-values in the upper part of each graph indicate significance level for the difference between UV-treated vs. across all seed lots. Individual seed lot comparisons with significance at P < 5 are indicated with.
Difference in Crown Grayscale (Sample 1) Difference in ear weight (Ear1) 3 2 1-1 -2-3 3 2 1-1 -2-3 P = 2 N N P = 1 P =.76 P =.33 P = 8 P = 6 N P = 36 N 14 12 1 8 6 4 2-2 Difference in Crown Grayscale (Sample 2).1 8 6 4 2-2 -4-6 -8 Difference in ear weight (Total weight) Figure 3. Field 1 -- Differences between plants grown from UV-treated seed and seed in crown grayscale at two sampling dates (sample 1 = 6 weeks after sowing; 2 = 12 weeks) and ear weights at maturity (UV treated Nontreated). P-values in the upper left corner indicate significance across all seed lots. P-values for each seed lot compared separately are reported in the bar if P <.1; any bar without a P-value is not significant at P =.1.
1.2 Node 1 N SCI P =.66 Nontreated UV N Crown P < 1 N P < 1 P < 1 PR N Figure 4. Field 2 -- Proportion of plants with crown, 1 st stalk node, subcrown-internode, or primary root infested with F. verticillioides 6 weeks after planting. P-values in the upper part of each graph indicate significance level for the difference between UV-treated vs. across all seed lots. Individual seed lot comparisons with significance at P < 5 are indicated with.
Crown N Node 2 P =.16 P = 29 N N Node 3 Node 1 P < 1 P = 79 Nontreated UV N Figure 5. Field 2 -- Proportion of plants with crown and stalk nodes (1 st through 3 rd above crown) infested with F. verticillioides 12 weeks after planting. P-values in the upper part of each graph indicate significance level for the difference between UV-treated vs. across all seed lots. Individual seed lot comparisons with significance at P < 5 are indicated with.
Difference in Crown Grayscale (Sample 1) Difference in ear weight (Ear1) 3 2 1-1 -2-3 -4-5 3 2 1-1 -2-3 -4-5 P =.52 N N P = 5 P = 3 P = 8 P = 3 P = 9 N P = 9 P = 11 P = 8 P = 15 N 2 15 1 5-5 -1 Difference in Crown Grayscale (Sample 2).15.1 5-5 -.1 -.15 Difference in ear weight (Total weight) Figure. Field 2. Differences between plants grown from UV treated seed and non-uv treated seed in crown grayscale at two sampling dates and weights at maturity (UV treated Nontreated). P-values in the upper left corner indicate significance across all seed lots. P-values for each seed lot compared separately are reported in the bar if lower than P =.1; All bars without a P-value are not significant at this level. Objective 2. Evaluate biological applications to sweet corn seed parents and subsequent Fusarium presence on silks and subsequent seed infection/contamination. Crown rot was present at high levels in but adventitious root rot was more severe than past years. treatments were associated with a trend towards fewer kernel infections by Fusarium species; this trend was not clearly present in sampling of silks. Kernels of the lines examined were treated with Apron Maxx RTA and then sown with a handpushed belt planter into our Fusarium crown rot field on the OSU Botany Field Lab (Electric Rd., Corvallis) on 28 June 21. Each corn line was replicated in seven 2-foot long rows.
Treatments included applications of MicroAF (an experimental biocontrol formulation) at 12.8 fl oz/a) as a soil drench about 2 weeks after planting and later as a spray during silking. Plants were irrigated every week with approximately 1.5 of water, depending on the weather conditions that week. Stand counts were made 3 weeks after sowing. Plants were evaluated at silking, 66-67 days after planting, for rot of roots, crown, and stalk nodes as well as Western Spotted Cucumber beetle larval feeding on roots. A plot code was used so that treatments were unknown to disease evaluators. Five plants were dug from each plot (35 plants total per treatment), soil was washed from the root balls of each plant, each internal crown was exposed by a longitudinal split through the lower stalk, and crowns were digitally scanned for crown grayscale analysis. Incidence of crown rot was also visually determined by C. Ocamb after splitting open each crown, noting whether crown rot was present, developing, or crown appeared healthy. Rot of roots, mesocotyl, and stalk nodes was determined visually for each plant. The rot of the primary root (radicle), adventitious root system, and subcrown-internode (mesocotyl) was visually estimated on a percentage basis while rot in the crown and stalk nodes as well as rootworm feeding was visually rated as follows: Nodal stalk rot rating = no discoloration of stalk nodes above crown 1 = node 1 above crown is discolored (dark brown) 2 = node 2 above crown is discolored (dark brown) 3 = node 3 above crown is discolored (dark brown) Crown rot rating = no discoloration of crown area (creamy-colored) or tan-light brown crown area (normal).5 = beginning of discoloration in crown area 1 = crown rot Root worm feeding = no root worm feeding is evident 1 = slight root worm feeding is evident 2 = < 75 % of adventitious roots at a single whorl have feeding 3 = > 75 % of adventitious roots at one whorl or > 5 % of adventitious roots at 2 whorls have feeding During the first week of October, ears were sampled from the 3 sets of inbred parents for 1861, 1477, and. Silks were excised in the laboratory and plated onto Fusarium-selective medium. During late November, one ear was harvested from each of five plants per plot for all three sets of seed parents. Ear weights were recorded and five kernels were excised from each ear and then plated onto Fusarium-selective medium. Stand was higher in hybrids than their respective parents and no effect was seen due to biocontrol treatment (data not shown). Disease was present in the plants when sampled at silking. Primary roots and mesocotyls were nearly entirely rotted.
Mean % primary root with rot Mean % primary root with rot 12 1 8 6 4 2 Mean % mesocotyl with rot 1 8 6 4 2 Mean % mesocotyl with rot -P1 -P2 -P1 -P2 -P1 -P2 -P1 -P2 -P1 -P2 -P1 -P2 Levels of adventitious root rot were higher overall this year than previous years, most likely due to the cooler conditions generally present during this growing season. Rootworm injury was present at slightly higher levels. Mean % adventitious roots with rot 1 8 6 4 2 Mean % adventitious roots with rot Mean rootworm injury 4 3 2 1 Mean rootworm injury -P1 -P2 -P1 -P2 -P1 -P2 -P1 -P2 -P1 -P2 -P1 -P2 Stalk nodes were rotted at least two nodes above the crown and sometimes were affected all the way up to the developing ear. Crown rot was present. Mean stalk node # discolored Mean stalk node # discolored -P1 -P2 -P1 -P2 -P1 -P2 8 6 4 2 Incidence of crown rot Incidence of crown rot -P1 -P2 -P1 -P2 -P1 -P2 1 8 6 4 2
Crown scans were done and differences were generally minimal between plants treated with a biocontrol formulation and those not treated. Fusarium species were recovered from silks of all treatments. more rot <<-----Mean crown grayscale 12 1 8 6 4 2 Mean crown grayscale -P1 -P2 -P1 -P2 -P1 -P2 Mean # Fusarium colonies/silk 4 3 2 1 Mean # Fusarium colonies/silk -P1 -P2 -P1 -P2 -P1 -P2 Ear weights were generally similar for the biocontrol and across the corn lines except for which saw a significant increase in average ear weights with biocontrol treatment. Hybrids ear weights were generally much higher then their respective seed parents. A trend was apparent for biocontrol-treated inbred ears to have fewer kernels with Fusarium species Mean ear wt (kg).5.3.1 Mean ear wt -P1 -P2 -P1 -P2 -P1 -P2 # kernels out of 5 with Fusarium 6 5 4 3 2 1 # kernels out of 5 with Fusarium -P1 -P2 -P1 -P2 -P1 -P2 2. Cooperate with other sweet corn projects within and outside of OSU. Ocamb evaluated sweet corn plants for rot of the primary and adventitious roots, mesocotyl, stalk nodes, and crown as well as crown grayscale for J. Myers (OSU Horticulture) and Rogers Brand Vegetable