Sensitivity of Cucurbita pepo Experimental Lines and Varieties to Phytophthora Crown Rot and Fruit Rot

Margaret Tuttle McGrath

Department of Plant Pathology, Cornell University, LIHRL, 3059 Sound Avenue,
Riverhead, NY 11901-1098

Additional index words. cucurbits, resistance, germplasm, breeding line

Abstract. A search for potential sources of resistance to phytophthora blight, which has become a major limitation to cucurbit production in some areas, was started in 1994 because of the unsatisfactory results being achieved with other control measures. In one series of experiments, variation in susceptibility to both fruit rot and crown rot caused by Phytophthora capsici was detected among Cucurbita pepo summer squash-type experimental lines and varieties. HMX 1708 was the overall best entry with a relatively low incidence of fruit rot, late onset of symptoms, and a low incidence of stem symptoms. In another series of experiments, Cucurbita pepo pumpkin-type experimental lines with hard, gourd-like rinds were shown to be less susceptible to fruit rot than pumpkins with standard rinds. Immature green fruit were susceptible, although phytophthora fruit rot is more often a problem late in the growing season.

 

Phytophthora fruit and crown rot (a.k.a., blight) has become a major limitation to cucurbit production in some areas. It is a relatively new disease in New York and 12 other states where it was first reported within the past 20 years (McGrath, 1996a). It is considered to be a serious problem and/or increasing in importance in 14 of the 29 states where it now occurs. Substantial losses have occurred because neither fungicides nor cultural practices provide adequate control when conditions are very favorable (Hausbeck and Kusnier, 1994, 1995; McGrath, 1994, 1995a, 1995b, 1995c, 1996b, 1996c, 1997, 1998; McGrath et al., 1993; McGrath and Shishkoff, 1996; McGrath and Staniszewska, 1994; Miller et al., 1995; Shishkoff and McGrath, 1998a, 1998b). Total crop loss has occurred in some fields.

The fungus Phytophthora capsici causes seedling damping-off, root and crown rot, stem lesions, foliar blight, leaf spots and fruit rot. All cucurbits are susceptible; pumpkin and squash seem to be affected most commonly, and cantaloupe are least susceptible. Crown rot causes the entire plant to completely collapse and die in a short period of time. Summer squash types often die back from the growing tip. The pathogen survives in soil for at least 2 years (McGrath, 1996a, 1996b).

A search for potential sources of resistance to phytophthora blight was started in 1994 because of the unsatisfactory results being achieved with other control measures. Two series of experiments were conducted. The objective of the first series was to evaluate Cucurbita pepo summer squash-type experimental lines and varieties. Experiments were conducted in 1994, 1995, and 1996. The objective of the second series was to evaluate pumpkins with hard, gourd-like rinds. Experiments were conducted in 1997 and 1998.

Materials and methods

Seed were obtained from T. H. Superak, Harris Moran Seed Company. All experiments were conducted at the Long Island Horticultural Research Laboratory in Riverhead, N.Y., in infested fields of Haven loam/Riverhead sandy loam where phytophthora blight had occurred on cucurbits at least one or two years previously. Seedlings were grown in a greenhouse, then acclimated to outdoor conditions for several days before transplanting. To provide conditions favorable for disease development, fields were irrigated excessively (> 2.5 cm/day) when the soil was already moist.

 

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Evaluation of C. pepo summer squash types. There were four or five replications in a randomized block design. Each plot consisted of a row of plants spaced 61 cm apart. There were four, seven and ten plants in 1994, 1995, and 1996, respectively. The spacing between rows was 173 cm. 'Supersett' was included in all experiments as a known susceptible variety for comparison. Plants and fruit were examined periodically for symptoms of fruit and crown rot. Fruit were removed from plants when they reached marketable size and left on the ground next to the plants. In 1994, there were two experiments located at opposite ends in areas of a field 500 ft (152.4 m) long considered to have relatively 'high' and 'low' disease pressure based on incidence of pumpkins with phytophthora fruit rot in 1992 and 1993. The 'high' disease pressure area of the field had a slightly lower elevation and the soil tended to drain more slowly after rain than the 'low' disease pressure area. There were not enough plants of SSXP 211 for it to be included in both experiments.

Evaluation of C. pepo pumpkin types. Plots consisted of two rows with seven plants each. A randomized complete block design with five replications was used. A row of yellow straight neck squash was planted through the center of the field to serve as a source of inoculum. Fruit were examined weekly for symptoms of phytophthora fruit rot and other diseases. Fruit were classified as being infected by Phytophthora only when sporangia were visible. Two healthy fruit of each experimental line were removed from each plot on 22 Oct. 1997. They were challenged with inoculum by putting them in enclosed plastic containers, misting them with water, then putting Phytoph

thora sporangia from a naturally infected pumpkin fruit on each one. Fruit that remained asymptomatic were rechallenged on 31 Oct. and 5 Nov.

Results and discussion

Evaluation of C. pepo summer squash types, 1994. Plants in the field appeared healthy until after 4 days of rain (a total of 8.5 cm fell on 14, 15, 20, and 22 Aug). Symptoms of phytophthora fruit rot were observed on all experimental lines and varieties on 27 Aug. 'Supersett' appeared to be substantially more susceptible than the other entries, but detecting this statistically was hampered because there was considerable variation in disease occurrence among replications within the two areas of the field (Table 1). In the 'low' disease pressure area, phytophthora fruit rot did not develop during the entire experiment in replication 1 while all entries in replication 4 had affected fruit on 27 Aug. Although 'Supersett' was infected in replication 3, the other entries remained asymptomatic. Many 'Supersett' plants died prematurely because of phytophthora crown rot. 'Aladdin' had the fewest plants with phytophthora fruit rot and the lowest percentage of affected fruit/plant in both experimental areas. SSXP 210 was evaluated previously by T. H. Superak and found to be resistant to root and crown rot in seedling tests. HMX 1708 had the lowest incidence of fruit rot when evaluated in New York in 1993.

 

Table 1. Sensitivity to phytophthora blight of Cucurbita pepo summer squash-type experimental lines and varieties in 1994.z

Plants with phytophthora (%) Fruit/plant with phytophthora (%)

Variety or Disease pressure (1992­93)

experimental line Low High All Low High All

Aladdin (Middle Eastern type) 31 ay 31 31 13 a 31 23

SSXP 210 (grey zucchini) 17 a 75 50 8 a 75 46

SSXP 211 (striped zucchini) --- 50 50 --- 50 50

HMX 1708 (grey zucchini) 50 ab 62 58 50 ab 62 58

Supersett (yellow crookneck) 92 b 100 96 66 b 89 79

P 0.0166 0.1044 0.0589 0.0518 0.1270 0.155

zMean percentage of plants/plot with Phytophthora fruit rot and mean percentage of affected fruit/plant were calculated for fruit examined between 27 Aug. and 6 Sept., when conditions were favorable for disease development, on four plants in each of four replications in each disease pressure area. These calculations do not include fruit that rotted for other reasons, plants that did not have fruit during this period, and replication 1 in the low disease pressure area where phytophthora fruit rot did not develop. Disease pressure was classified as relatively high and low in the two experimental areas based on incidence of pumpkins with Phytophthora fruit rot in 1992 and 1993.

yMeans followed by the same letter in a column are not significantly different according to Fisher's protected lsd.

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Table 2. Sensitivity to phytophthora blight of Cucurbita pepo summer squash-type experimental lines and varieties in 1995.

First week

Variety or Fruit with phytophthora fruit rot (%) fruit rot Plants with

experimental line Aug. Sept. Total was seen symptoms (%)

Aladdin 4.7 23.4 az 14.6 2.2 a 50.6 bcd

HMX 1708 0.0 33.5 ab 15.3 3.8 c 12.5 ab

Jason 0.0 42.1 ab 16.0 3.5 c 48.3 abcd

SSXP 286 2.5 45.2 abc 18.0 3.0 abc 41.1 abcd

SSXP 287 14.3 29.9 ab 18.9 2.5 a 62.5 cd

SSXP 210 1.0 52.8 bc 24.2 3.0 abc 4.2 a

Genie 14.8 52.4 bc 24.2 2.5 a 25.0 abc

SSXP 285 12.3 71.1 cd 30.7 2.8 ab 73.2 d

Zucchini Elite 18.0 71.1 cd 33.4 2.5 a 50.0 bcd

SSXP 288 14.4 52.8 cd 34.9 3.0 abc 36.1 abcd

Supersett 11.2 81.8 d 35.8 2.5 a 75.0 d

P 0.2775 0.0034 0.1235 0.0787 0.0542

zMeans followed by the same letter in a column are not significantly different according to Fisher's protected lsd.

Evaluation of C. pepo summer squash types, 1995. Phytophthora fruit rot was observed first on 21 Aug., 4 days after the field was irrigated excessively. Symptoms occurred in only five plots: one plot each of 'Genie', SSXP 210, SSXP 287, SSXP 288, and 'Zucchini Elite'. Incidence was higher on 28 Aug. following excessive irrigation on 22 and 23 Aug. Symptoms were not observed on any plants of HMX 1708, 'Jason', or SSXP 210 on 28 Aug. Affected fruit often were found on plants that otherwise appeared healthy. For example, fruit with Phytophthora were observed on 49 plants while only six plants had symptoms of crown rot on 28 Aug. Supersett was very susceptible to Phytophthora. In addition to the high percentage of fruit that developed symptoms during September, many plants died because of crown rot. On 14 Sept., 46% of 'Supersett' plants were dead because of crown rot while 0% to 22% of plants of other entries in this experiment had died. The type of stem symptom on the other entries often was necrosis of the growing tip, which sometimes progressed such that the entire plant

collapsed and died. 'Aladdin' had the lowest percentage of fruit with symptoms during this experiment (Table 2). This variety also performed well in 1994. However, 50% of 'Aladdin' plants developed crown rot. In comparison with 'Aladdin', significantly fewer SSXP 210 plants developed stem symptoms whereas the percentage of fruit with symptoms was significantly higher. SSXP 210 had a relatively low incidence of fruit rot in 1994. HMX 1708 was the overall best entry with a relatively low incidence of fruit rot, late onset of symptoms, and a low incidence of stem symptoms. This experimental line also performed well in 1993. Results generally were as expected based on previous results from greenhouse seedling tests conducted by T. H. Superak and/or pedigree information, except that SSXP 286 was very susceptible and SSXP 285 was resistant to crown rot in seedling trials.

Table 3. Sensitivity to phytophthora blight of Cucurbita pepo summer squash-type experimental lines and varieties in 1996.

Fruit with phytophthora fruit rot (%) Plants with

Variety or On plants Detached stem symptoms (%)

experimental line 27 Sept. Totalz 27 Sept. Totalz 27 Sept. Totalz

Aladdin 16.7 cy 18.1 d 35.3 c 43.3 de 13.9 e 64.5 c

SSXP 210 42.1 abc 56.5 abc 14.9 d 26.7 e 12.0 e 32.4 e

HMX 1708 22.5 bc 42.8 bcd 39.2 c 62.8 bc 14.7 de 31.7 e

Moctezuma 28.8 bc 34.9 cd 32.9 cd 52.5 cd 26.9 bcde 37.9 de

SSXP 211 18.1 c 40.0 bcd 45.4 bc 64.4 bc 25.8 cde 46.7 cde

SSXP 288 27.0 bc 37.7 cd 38.3 c 53.0 cd 37.4 bc 66.8 bc

HMX 5729 23.3 bc 40.3 bcd 36.6 c 53.0 cd 25.0 cde 43.1 de

Jason 20.8 bc 35.8 cd 36.7 c 61.7 bc 36.4 bcd 56.7 cd

Zucchini Elite 46.1 ab 64.9 ab 65.8 a 81.6 a 48.6 ab 87.3 ab

Supersett 62.4 a 79.2 a 60.8 ab 74.9 ab 69.7 a 100.0 a

P 0.0361 0.0023 0.0008 0.0001 0.0001 0.0001

zTotals are for 27 Sept. through 11 Oct.

yMeans followed by the same letter in a column are not significantly different according to Fisher's protected lsd.

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Table 4. Sensitivity to phytophthora fruit rot of Cucurbita pepo pumpkin-type experimentals in 1997.

Fruit with phytophthora (%) Healthy fruit (%) Fruit with phytophthora

Experimental line (15 Sept.­5 Nov.)z (5 Nov.) rot from inoculation (%)

HMX 4696 2 by 97 a 0

HMX 5682 2 b 93 a 33

HMX 4695 25 a 64 b 78

HMX 2692 42 a 50 b 100

P 0.0004 0.0001

zTotal observed with phytophthora over this time period.

yMeans followed by the same letter in a column are not significantly different according to Fisher's protected lsd.

Evaluation of C. pepo summer squash types, 1996. Symptoms were first observed on 25 Sept., 1 week after 6.9 cm of rain fell over two days. Symptoms were observed in all replications. No symptoms were observed on 27 Sept. on 'Aladdin', 'Moctezuma', SSXP 210, SSXP 288, or HMX 5729 in replication 1, where disease pressure was lowest. Necrosis of growing tips was the most common stem symptom. Stem symptoms progressed rapidly on Supersett: most affected plants were collapsing by 4 Oct. and dead by 11 Oct. Percentage of plants collapsing or dead on 4 Oct. was 78% for 'Supersett'; 51% for 'Zucchini Elite'; 0% to 9% for SSXP 210, HMX 1708, 'Moctezuma', and HMX 5729; and 15% to 33% for the other varieties and experimental lines . Leaf spots developed on all experimental lines and varieties. 'Supersett' and 'Zucchini Elite' were the most susceptible; they did not differ significantly in any of the variables analyzed. 'Aladdin' was the least susceptible; it was the only one with significantly lower incidence of phytophthora fruit rot and stem symptoms compared with Supersett for all six variables in the table (Table 3). This variety also performed well in 1994 and 1995. Incidence of phytophthora fruit rot on detached fruit and incidence of stem symptoms was low for SSXP 210; however, incidence of fruit rot on plants was not significantly different from 'Supersett'.

Evaluation of C. pepo pumpkin types. Substantial differences in susceptibility to Phytophthora were detected. The two experimental lines with hard rinds (HMX 4696 and HMX 5682) were much less susceptible than the other two experimental lines tested (Table 4). In 1997, symptoms were first observed on 15 Sept. in HMX 4695 and on 22 Sept. in HMX 2692, but not until 5 Nov. in HMX 4696 and HMX 5682. HMX 4696 and HMX 5682 also appeared to be less susceptible when challenged with inoculum. In 1998, symptoms were first observed on 25 Aug. in HMX 4695, 8 Sept. in HMX 2692, and 18 Sept. in HMX 4696 and HMX 5682. Immature fruit of the hard-rinded experimental lines seemed to be as susceptible as those of standard-rinded experimental lines, however, phytophthora fruit rot is more often a problem late in the growing season when few immature fruit are present.

Conclusion

Differences in susceptibility to Phytophthora were detected that potentially could be useful for managing phytophthora blight. It is anticipated that resistant varieties will need to be used as part of an integrated management program to successfully control phytophthora blight, especially when disease pressure is high.

Literature cited

Hausbeck, M.K. and J.J. Kusnier, III. 1994. Evaluation of fungicides for managing phytophthora crown and fruit rot in spaghetti squash, 1993. Fungicide Nematicide Tests 49:147.

Hausbeck, M.K. and J.J. Kusnier, III. 1995. Evaluation of fungicides for managing phytophthora crown and fruit rot in spaghetti squash, 1994. Fungicide Nematicide Tests 50:156.

 

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McGrath, M.T. 1994. Fungicides provided insufficient suppression of phytophthora fruit rot of cucurbits when disease pressure was high. Phytopathology 84:1373 (abstr.).

McGrath, M.T. 1995a. Evaluation of fungicides for managing phytophthora crown rot and fruit rot in pumpkin, 1994. Fungicide Nematicide Tests 50:145.

McGrath, M.T. 1995b. Evaluation of fungicides for managing phytophthora fruit rot in pumpkin based on results from challenge inoculation, 1994. Fungicide Nematicide Tests 50:148.

McGrath, M.T. 1995c. Evaluation of yard-waste compost used along or in combination with fungicides for managing phytophthora crown rot and fruit rot in pumpkins, 1994. Biol. Cult. Tests 10:143.

McGrath, M.T. 1996a Phytophthora fruit rot, p. 53­54 In: T.A. Zitter, D.L. Hopkins, and C.E. Thomas (eds.). Compendium of cucurbit diseases. APS Press, St. Paul., Minn.

McGrath, M.T. 1996b. Two-year rotation used alone or combined with a fungicide program for managing phytophthora fruit rot in pumpkin, 1995. Fungicide and Nematicide Tests 51:142.

McGrath, M.T. 1996c. Yard-waste compost provided insufficient suppression of phytophthora fruit rot of cucurbits. Phytopathology 86:S123­124.

McGrath, M.T. 1997. Evaluation of solarization and sorghum sudan grass for managing phytophthora crown rot and fruit rot in pumpkin, 1995­1996. Biol. Cult. Tests 12:159.

McGrath, M.T. 1998. Evaluation of oat straw mulch and ryegrass living mulch for managing phytophthora crown rot and fruit rot in pumpkin, 1997. Biol. Cult. Tests 13:174.

McGrath, M.T., M.S. Ghemawat, and H. Staniszewska. 1993. Evaluation of fungicides for managing phytophthora crown rot and fruit rot in pumpkin under stringent conditions, 1992. Fungicide Nematicide Tests 48:172.

McGrath, M.T. and N. Shishkoff. 1996. Evaluation of yard-waste compost used alone or in combination with fungicides for managing phytophthora crown rot and fruit rot in pumpkin, 1995. Biol. Cult. Tests 11:115.

McGrath, M.T. and H. Staniszewska. 1994. Evaluation of fungicides for managing phytophthora crown rot and fruit rot in pumpkin, 1993. Fungicide Nematicide Tests 49:141.

Miller, S.A., R.M. Riedel, T. Anderson, and A.M. Denning. 1995. Evaluation of fungicide treatments for control of phytophthora blight of squash, 1994. Fungicide Nematicide Tests 50:160.

Shishkoff, N. and M.T. McGrath. 1998a. Evaluation of cocoa shell, oat straw and black plastic mulch for managing phytophthora fruit rot in pumpkin, 1997. Biol. Cult. Tests 13:173.

Shishkoff, N. and M.T. McGrath. 1998b. Evaluation of fungicides for phytophthora fruit rot and tip blight of summer squash, 1997. Fungicide Nematicide Tests 53:194.

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