Crop Loss to Eight Diseases of Watermelon in North Carolina

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Cucurbit Genetics Cooperative Report 22:47-49 (article 17) 1999

Nischit V. Shetty and Todd C. Wehner
Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609

The Department of Plant Pathology at North Carolina State University set up a disease loss committee in 1978 for crops grown in North Carolina, in response to inquiries made by the National Agricultural Pesticide Impact Assessment Program. Crop coordinators, in consultation with colleagues having knowledge of the crops, were responsible for arriving at estimates of disease incidence (percentage of the crop production area affected by diseases including nematodes) and percentage reduction in crop value for North Carolina. Reports were generated from 1979 through 1988, when the program was discontinued. Data on watermelon (Citrullus lanatus (Thunb,) Matsum. & Nakai) are available, so we decided to analyze the most recent six years (1, 2, 3, 4, 5, 6) to identify the most important watermelon pathogens based on their incidence in North Carolina.

The data from the reports reviewed for the six years (1983 through 1988) were estimated from research plots, sample surveys and using the opinions of extension specialists and county agents. Each estimate was assigned to a confidence rating (1 to 3) indicting how the data was obtained and its reliability. a rating of 1 indicated ‘confident’ (actual disease measurements made through surveys or research tests). A rating of 2 indicated ‘reliable’ (estimates based on knowledge of the crop in relation to the general distribution and severity of the disease). a rating of 3 indicated ‘indicative’ (estimates were an educated guess).

Eight diseases were reported during the six years of the survey for watermelon. the estimated incidence of most pathogens remained constant over the five years (Table 1). Blossom end rot caused by Choanephoba cucurbitarum had the highest incidence of all pathogens for all six years, followed by damping off, gummy stem blight (Didymella bryoniae (Auersw.) Rehm), and root knot (nematodes of Meloidogyne spp.). The incidence of the above four diseases as well as rind necrosis was constant over the six years. However, the incidence of anthracnose (Colletorichum orbicularae Berk & Mont.) Arx) increased over years. Data for stem rot incidence was available only for 1988, with a disease incidence rating of 75. The data for all years were of rating class 3.

Based on the percentage loss of crop value, root-knot nematodes caused the greatest loss over the six years, accounting for nearly one third of all disease losses. The other important diseases causing crop losses were gummy stem blight and blossom end rot. Since the loss to root knot decreased in 1987 and 1988, and the loss to gummy stem blight remained constant, gummy stem blight was the most important disease in 1987 and 1988 (Table 2).

Root knot nematode cost the most in terms of dollars lost (including prevention costs), followed by gummy stem blight and blossom end rot. Dollars lost (including prevention costs) due to gummy stem blight increased over the six year period. Similar trends were also observed for blossom end rot, anthracnose and downy mildew (Table 3).

Table 1. Estimates incidence of eight diseases on all field-grown watermelons in North Carolina (1983 to 1988).

Rank Disease 1983 1984 1985 1986 1987 1988 Mean
1 Blossom end rot 80 80 80 80 80 80 80
2 Damping off 70 70 70 70 70 70 70
3 Gummy stem blight 70 70 70 70 70 70 70
4 Root knot nematode 50 50 50 50 50 50 50
5 Anthracnose 10 10 25 25 25 25 20
6 Downy mildew – – – 20 20 20 20
7 Rind necrosis 15 15 15 15 15 15 15
8 Leaf Spot 10 10 10 10 10 10 10
Mean 44 44 46 43 43 43 42

Table 2. Estimated percentage loss in crop value caused by eight diseases on field-grown watermelons in North Carolina (1983 to1988).z

Rank Disease 1983 1984 1985 1986 1987 1988 Mean
1 Blossom end rot 10.0 10.0 10.0 10.0 5.0 5.0 8.3
2 Damping off 7.0 7.0 7.0 7.0 7.0 7.0 7.0
3 Gummy stem blight 4.0 4.0 4.0 4.0 4.0 4.0 4.0
4 Root knot nematode 2.1 2.1 2.1 2.1 2.1 2.1 2.1
5 Anthracnose 1.0 1.0 2.5 2.5 2.5 2.5 2.0
6 Downy mildew – – – 2.0 2.0 2.0 2.0
7 Rind necrosis 0.5 0.5 0.5 0.5 0.5 0.5 0.5
8 Leaf Spot 0.3 0.3 0.3 0.3 0.3 0.3 0.3
Totals 24.9 24.9 26.4 28.4 23.4 23.4 26.2

z Estimated percentage loss of value calculated as the percentage dollar loss due to yield and quality reduction.

Table 3. Estimated cost (1000 dollars) of control (loss due to yield and quality reduction, and cost of prevention and control) of eight diseases on field-grown watermelons in North Carolina (1983 to 1988).z

Rank Disease 1983 1984 1985 1986 1987 1988 Mean
1 Blossom end rot 265 773 616 89 594 867 4,006
2 Damping off 158 544 409 600 768 1,137 3,616
3 Gummy stem blight 75 284 224 337 431 639 1,990
4 Root knot nematode 34 118 158 221 283 418 1,232
5 Anthracnose 38 149 117 177 227 336 1,043
6 Downy mildew – – – 179 229 338 746
7 Rind necrosis 25 83 46 53 67 98 372
8 Leaf Spot 5 21 17 25 32 48 149
Totals 600 1,972 1,587 2,482 2,632 3,882 13,155

z Estimated cost calculated as the total dollar loss due to yield and quality reduction and cost of prevention and control by eight pathogens by years.

Table 4. Coefficients of determination (R2) for estimated incidence, estimated percentage loss of crop value, estimated cost, and year of estimate for watermelons due to eight diseases in North Carolina 1983 to 1988).

Variable Incidencez LossY CostX
Year 0.988 0.945 0.922
Incidence 0.038 -0.023
Loss 0.956

z Estimated incidence calculated as the average incidence of eight pathogens by year.
y Estimated percentage loss of value calculated as the total dollar loss due to yield and quality reduction by eight pathogens by years.
x Estimated cost calculated as the total dollar loss due to yield and quality reduction and cost of prevention and control by eight pathogens by year.

Literature Cited

  1. Main, C.E., S.M. Nusser, and A.W. Bragg (eds.). 1984. 1983 crop losses in North Carolina due to plant diseases and nematodes. Dept. Plant Pathology Special Publication No. 3, North Carolina State University, Raleigh NC 146 pp.
  2. Main, C.E. and S.M. Nusser (eds.). 1985. 1984 estimates of crop losses in North Carolina due to plant diseases and nematodes. Dept. Plant Pathology Special Publication No. 4, North Carolina State University, Raleigh, NC 152 pp.
  3. Main, C.E. and S. V. Byrne (eds.). 1986. 1985 estimates of crop losses in North Carolina due to plant diseases and nematodes. Dept. Plant Pathology Special Publication No. 5, North Carolina State University, Raleigh NC 183 pp.
  4. Main, C.E. and S.V. Byrne, (eds.). 1987. 1986 estimates of crop losses in North Carolina due to plant diseases and nematodes. Dept. Plant Pathology Special Publication No. 6, North Carolina State University, Raleigh, NC 183 pp.
  5. Main, C.E. and S. K. Gurtz (eds.). 1988. 1987 estimates of crop losses in North Carolina due to plant diseases and nematodes. Dept. Plant Pathology Special Publication No. 7, North Carolina State University, Raleigh, NC 209 pp.
  6. Main, C.E. and S.K. Gurtz (eds.). 1988. 1987 estimates of crop losses in North Carolina due to plant diseases and nematodes. Dept. Plant Pathology Special Publication No. 8, North Carolina State University, Raleigh, NC 209 pp.