Fungicide resistance and host susceptibility of Colletotrichum orbiculare infecting cucurbit

crops in North CarolinaNicholas Noël and Lina M. Quesada-Ocampo, PhD

Department of Plant Pathology, North Carolina State University, Raleigh, NC

Introduction

1. Evaluate commercially available watermelon cultivars for their agronomic traits and disease resistance in the field and greenhouse.

2. Characterize C. orbiculare populations infecting cucurbit crops in NC for fungicide resistance and virulence.

Results

Results

Objectives Conclusions and Future Directions

Acknowledgements

Methods

In vitro FungicidesIn North Carolina, watermelon (Citrullus lanatus) production for the 2015 growing season was valued in excess of $32.3 million1. The eastern and northeastern parts of the state are ideal for production, where the Coastal Plain provides rich and diverse soil for agriculture. As of 2015 North Carolina produces 6% of the US’s watermelons, place the state 6th in total value of US production2.

Cucurbit anthracnose, caused by the ascomycete Colletotrichum orbiculare syn. lagenarium, is a widespread fruit and foliar disease that results in yield losses across the United States. In North Carolina, favorable environmental conditions, host availability, and long growing seasons offer ample opportunity for disease development, resulting in significant economic losses. Application of effective fungicides and planting of resistant cultivars, along with appropriate cultural practices, have proven effective in reducing anthracnose.

Isolates of C. orbiculare were obtained from collaborators in South Carolina, Indiana, Maryland, and Florida or through submissions to the NCSU PDIC. All isolates were grown on PDA, placed into long-term storage and revived on PDA.

Agar plugs (7 mm in diameter) were cut from the edge of actively growing cultures grown on PDA for 10 days. Plugs were placed onto fungicide-amended media and were incubated at 24 ± 1°C under a 16-hour photoperiod. After 7 days of growth, two perpendicular colony diameters per plate were measured and averaged. The diameter of the agar plug was subtracted from the colony diameters before calculating means3.

C. orbiculare isolates were obtained from infected leaves, stems, and fruit on watermelon samples submitted to the Plant Disease and Insect Clinic at NC State University.

36 cultivars were selected on the basis of their diseases resistance and their popularity among growers in NC. Plots were designed to optimize disease pressure equally across all cultivars and plots were replicated 4 times at Cherry Research Farm in Goldsboro, NC.

1. 23 lines showed less than 10% disease severity and incidence indicating that anthracnose resistance is available in commercial cultivars.

2. Initial fungicide resistance evaluations indicate sensitivity to Pyraclostrobin, but insensitivity to Chlorothalonil was observed in 1 isolate.

3. Field and greenhouse evaluations will be replicated next year and fungicide resistance evaluation will be expanded to 100 diverse US isolates.

We thank all members of the Quesada Lab, Shawn Butler from the NCSU PDIC, the Cherry Farm, Extension Agents, and grower cooperators. We also thank Anthony Keinath (Clemson), Dan Egel (Purdue), Fanny Iriarte (Florida), Kathryne Everts (Maryland), Bhabesh Dutta (Georgia), and the ATCC for providing isolates. Funding for this work was provided by USDA-SCRI CucCAP.

Variety Trial

Methods

Greenhouse seeding occurred on 4/19/16 and transplanted on 5/20/16. Isolates were grown on V8 agar at 24 ± 1°C under a 16-hour photoperiod. Conidia were harvested, filtered through cheese cloth, and concentrated to a final application rate of 50,000 spores/ plot. Inoculations occurred on 6/14, 6/30, 7/5, 7/28 via Solo backpack sprayer. Disease ratings were taken on 7/9, 7/16, and 7/24. Fruit was harvested and graded on 7/26.

A B C

A B

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References1. USDA-NASS, 2016; State Agriculture Overview: North Carolina2. USDA-NASS, 2016; Vegetable 2015 Summary3. Keinath, AP. 2015. Baseline Sensitivity of Didymella bryoniae to Cyprodinil and

Fludioxonil and Field Efficacy of these Fungicides Against Isolates Resistant to Pyraclostrobin and Boscalid. Plant Dis. 99:-822

Figure 2: A, Congo, a resistant cultivar, shows low disease incidence and severity; B, AU Producer, tolerant to Race 1 and Race 2, shows medium disease incidence and severity; C, Moon & Stars, a susceptible cultivar, shows high disease incidence and severity; D, Sorbet F1, with intermediate resistance to anthracnose, shows the highest disease incidence and severity.Table 1: Selected cultivars for field and greenhouse trials.Figure 3: A, Average disease ratings over three rating periods across four replicates; B, Average fruit yield and weight across four replicates; C, Sensitivity of South Carolina isolates to recommended fungicides.

Figure 1: A, Anthracnose symptoms on watermelon, depicting characteristic lesions; B, Colletotrichum orbiculare on PDA after 7 days incubation at 24 ± 1°C under a 16-hour photoperiod; C, Conidia harvested from V-8 agar via Tween-20/water solution.

SCCO12 (Race 1) SCCO14 (Race 2) SCCO15 (Race 2B) SCCO17 (Race 2B) SCCO18 (Race 2B)

D

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Figure 3: Disease ratings

Cultivar Number (Table 1)

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ease

(%)

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Table 1: Selected Cultivars for Field Trial

Plot Variety Seeded Resistance Race

#1 Congo Seeded Resistant Anth

#2 Black Diamond Seeded N/A N/A

#3 Vista Seeded Tolerant Anth

#4 Bold Ruler Seedless N/A N/A

#5 SSX 8585 Seeded High Resistance Anth1

#6 Carolina Cross Seeded N/A N/A

#7 Estrella Seeded Intermediate R Anth1

#8 Dixie Queen Seeded N/A N/A

#9 Mardis Gras Seeded Intermediate R Anth1

#10 Florida Giant Seeded N/A N/A

#11 Top Gun Seeded Intermediate R Anth1

#12 Jubilee Seeded N/A N/A

#13 Jubilee II Seeded N/A N/A

#14 Au Producer Seeded Tolerant Anth1, Anth2

#15 Mickylee Seeded N/A N/A

#16 SV0241WA Seedless High Resistance Anthracnose

#17 Moon & Stars Seeded N/A N/A

#18 Sorbet F1 Seedless Intermediate R Anthracnose

#19 Provider Seedless N/A N/A

#20 Travaler F1 Seedless Intermediate R Co

#21 Tri-X 313 Seedless N/A N/A

#22 Troubadour F1 Seedless Intermediate R Co

#23 Unbridled Seedless N/A N/A

#24 Royal Sweet Seeded High Resistance Co1

#25 Starbrite Seeded High Resistance Co1

#26 Yellow Buttercup Seedless N/A N/A

#27 720 Seeded Intermediate R Co1

#28 Solitaire Seedless N/A N/A

#29 7387 Seedless Intermediate R Co1

#30 Tendersweet Orange Seeded N/A N/A

#31 Fantasy Seeded Intermediate R Co1

#32 Sangria Seeded Intermediate R Co1

#33 Melody Seedless Intermediate R Co1

#34 Secretariat Seedless N/A N/A

#35 Ruby Seedless Intermediate R Co1,3

#36 Sweet Delight Seedless N/A N/A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 360

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55 57 59

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Figure 3: Average Fruit Count and Weight Total FruitAverage Fruit Weight (lbs)

Cultivar Number (Table 1)

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ght (

lbs)

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Figure 3: Fungicide sensitivity of Colletotrichum orbiculare

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