Cucurbit Genetics Cooperative Report 15:93-96 (article 36) 1992
T. A. Zitter and M. M. Kyle
Departments of Plant Pathology and Plant Breeding, Cornell University, Ithica, NYU 14853
Powdery mildew (PM) caused by Sphaerotheca fuliginea and gummy stem blight (GSB) / black rot caused by Didymella bryoniae (teleomorph) and Phoma cucurbitacearum (anamorph) are important diseases of many cucurbits. Growers in New York and other regions of the northeast have frequently complained about the collapse of pumpkins in the field or during the fall marketing period. Loss of handles (peduncles) at the same time is also a common occurrence. GSB is particularly important because plants are already infected with PM and are predisposed to more severe GSB (1). There are no reports on the potential interaction of PM and GSB susceptibility leading to collapse of pumpkins caused primarily by black rot. In 1990 and 1991, field studied were initiated to examine the rate, timing, and fungicide combinations necessary to control PM and GSB in pumpkin (cv. Howden). This variety was chosen because market losses occur more commonly with large pumpkins. In 1991 we also had the opportunity to evaluate two segregating pumpkin breeding lines with PM resistance, with Howden used in the backcross. This report summarizes the results of postharvest studies to establish the importance of PM and GSB on crop losses.
Methods. An isolate of D, bryoniae was recovered from infected muskmelon leaves showing typical symptoms of GSB in Onondaga County, NY in 1990. This isolate was maintained on V-8 agar plates at 21 C with 14 hr light. The concentration of the spore suspension was adjusted to 6 X 104 with nutrient solution (1), and was used to inoculate the segregating pumpkin breeding lines on 24 July 1991. A greenhouse isolate of S. fuliginea accidentally infected all cucurbit seedlings transplanted to the field in 1991, thus allowing for early occurrence of PM in plots used for chemical control and evaluation of the segregating pumpkin lines. Both PM and GSB occurred naturally in the plots in 1990. In 1991, the pumpkin plots were on the same land used the previous year to increase the natural occurrence of GSB.
The details of the fungicide trials conducted at Freeville, NY in 1990 and 1991 have been reported (2, 3). A total of 48 pumpkins were held in a walk-in cooler set at 55 F for 6 wks in 1990, while in 1991 63 fruit were held for 12 wks.
Two early generation pumpkin lines backcrossed to Howden and segregating for PMR (9-741-2 X 747), referred to as line 741, and (9-746-1 X 747-4), referred to as line 746, were produced. Plants already infected with PM were transplanted to the field on 4 June and subsequently inoculated with the GSB organism on 24 July. Individual plants were rated for PMR segregation, and the severity of disease noted in June and July prior to GSB occurrence. The PM rating scale used was 0 = no colonization, 1 = mild infection on both leaves and stems, 2 = moderate infection, and 3 = severe infection. Individual pumpkins with good appearance and dark green handles were harvested 19 Sept. and stored in the same cooler as mentioned above for 11 wks. The organisms responsible for collapse were determined in the laboratory.
Results. Disease pressure was reasonably high in 1990 for both PM and GSB as indicated by mid-season disease rati8ngs (Table 1). Pumpkin collapse began 1 mo into storage and progressed rapidly. Black rot lesions were evident on most collapsing pumpkins, with a water-soaked, circular lesion surrounding the blackening tissue. The inside of the pumpkin was often filled with water. The greatest losses occurred most frequently for those treatments which provided poor control of both PM and GSB. Good control is usually achieved with a broad spectrum fungicide like Bravo and systemic control of PM provided by Bayleton or Beniate.
In 1991, PM pressure was intense owing to the early introduction of inoculum, but because the season was reasonably dry, GSB was not readily detected in the foliar blight phase. This lack of severe pressure is reflected in the ability to hold most of the stored pumpkins for 11 wk (Table 2). Greatest losses again were sustained in those fungicide treatments which provided less than adequate control of either PM or GSB or both diseases. The produce ASC at both rates provided systemic control of PM, giving results comparable to the Bravo + Beniate treatment. Anthracnose, although not as common as black rot, did appear as small sunken spots usually occurring on the side of the rind facing the ground.
In the field it was observed that line 741 was less susceptible to PM, while line 746 showed considerable loss of foliage attributed to both PM and GSB. A good correlation existed between plant susceptibility to PM that resulted in fruit losses primarily due to GSB infections (Table 3). Nine of the 20 fruit discarded from line 746 showed watery collapse of the rind due to black rot. Only three pumpkins were discarded for line 741 with black rot responsible in two cases and anthracnose in the other. The remaining pumpkins were still firm with dark green handles. In all cases, discarded fruit originated from plants which had earlier been rated in the field at 2 or greater for PM severity.
The two separate lines of evidence support the conclusion that susceptibility to PM does predispose plants to greater levels of infection by GSB, and as a result losses in the field or in storage from black rot can be expected. Control of PM through resistance should significantly reduce market diseases and storage losses.
Table 1. Evaluation of fungicides for powdery mildew (PM) and gummy stem blight (GSB) control in Howden pumpkin, 1990a
Treatment & rate /A (spray interval in days) |
Disease PM |
ratingb GDB |
Storage lossesc |
| Bravo 720 2.0 pt (7) | 2.8 | 2.0 | 2/6 |
| Bravo 720 3.0 pt (7) | 2.8 | 1.3 | 3/6 |
| Bravo 720 2.0 pt (7) + Bayleton 50 DF 4 oz (14) | 2.3 | 1.0 | 0/6 |
| Bravo 720 3.0 pt (7) + Bayleton 50 DF 4 oz (14) | 2.3 | 1.3 | 0/6 |
| Bayleton 50 DF 2 oz (14) | 2.3 | 1.5 | 5/6 |
| Bayleton 50 DF 2 oz (14) + 0.75 JMS Stylet Oil (14) | 3.3 | 1.2 | 1/6 |
| Bayleton 50 DF 4 oz (14) | 3.2 | 1.2 | 3/6 |
| Untreated Check | 4.0 | 1.7 | 4/6 |
a No significant difference in yield (RCB with 3 replications). PM and GSB occurred naturally in plots.
b Foliar rating: 1 = 20% disease; 2 = 21-40% disease; 3 = 41-60% disease; 4 = 61-80% disease; 5 = over 80% disease.
c Six pumpkins were selected at random (2 per rep and treatment) and stored at 55 ˚ F for 6 wk.
Table 2. Effect of fungicide treatments in Howden pumpkins on handle appearance and storage losses, 1991
Treatment |
Handle appearance and commentsa |
Losses band disease |
|||
(Spray interval in days) |
Firm |
Soft |
Shr/Disc |
Wh. Myc. |
|
| Bravo (7) | 1 | 6 | 2 | (4) | 2 (2 BR) |
| Bravo (7) + Bayleton (14) | 4 | 3 | 2 | (4) | 3 (2 BR, 1A) |
| Bravo (7) + Benlate (7) | 5 | 3 | 1 | (4) | 0 |
| Bravo (7) + ASC-low (14) | 5 | 3 | 1 | (3) | 1 (BR) |
| Bravo (7) + ASC-high (14) | 3 | 4 | 2 | (4) | 0 |
| KHCO3 + Sun Spray Oil (7) | 1 | 8 | 0 | (7) | 4 (2 BR, 1 A, 1 A + BR) |
| Water Control | 3 | 4 | 2 | (4) | 2 (2 BR) |
a Nine pumpkins selected (3/rep) had good appearance on 9/13/91. Handle readings made 1 mo. after storage at 55˚F. Rind appearance was good. Handles noted as firm, soft, shriveled and discolored, with white mycelia growth noted.
b Pumpkins were discarded over a 12 wk period with most occurring on NOv. 15 and Dec. 6 (experiment ended). BR = Black rot caused by Didymella bryoniae (Gummy stem blight); A = Anthracnose caused by Colletotrichum orbiculare.
Table 3. Reaction of segregating PMR pumpkin lines for powdery mildew and gummy stem blight, 1991
Damage by hilla |
No. fruit |
||||||
Pumpkin lines |
Rating: |
0 |
1 |
2 |
3 |
Storage resultsb |
|
| 741 x Howden | 10 | 6 | 2 | 3 | 21 | 3 discarded (2 BR, 1 A) | |
| 746 x Howden | 0 | 15 | 2 | 3 | 20 | 9 discarded (9 BR) | |
a PM rating 0 + no mildew on leaves or stems, 3 = heavy colonization.
b Moved to storage 19 Sept at 55 ˚ F. Final readings 6 Dec. BR = Black rot caused by Didymella bryoniae (gummy stem blight), A = Anthracnose caused by Colletorichum orbiculare.
Literature Cited
- Bergstrom, G.C., D.E. Knavel, and J. Kuc. 1982. Role of Insect Injury and powdery mildew in the epidemiology of the gummy stem blight disease of cucurbits. Plant Dis. 66:683-686.
- Zitter, T.A. and L. Hsu. 1991. Evaluation of fungicides for control of powdery mildew of pumpkin, 1990. Fungicide and Nematicide Tests 46:167.
- Zitter, T.A.and L. Hsu. 1992. Evaluation of fungicides for control of powdery mildew of pumpkin, 1991. Fungicide and Nematicide Tests 46: (in press).