Evaluation of New Acorn Squash Varieties Resistant to Powdery Mildew, 2020

This replicated experiment was conducted at LIHREC in 2020.

Topics on this page:

• Procedures
• Results
• Tables
• Images

Powdery mildew (PM) is the most important disease affecting cucurbit crops, occurring every year throughout Long Island as well as the rest of the U.S.  Management is needed to avoid loss in yield and especially fruit quality.  Flavor of melons and squashes is poor and they lack sweetness when leaves senesce prematurely due to a disease like PM not being successfully managed.  Fungicides and PM resistant (PMR) varieties are the only management tools for PM.  Using an integrated program with fungicides applied to PMR varieties is recommended to minimize selection pressure on the pathogen to develop resistance.  This pathogen has proven adept at developing resistance, especially to targeted fungicides.  PMR cucurbit varieties are an important tool for managing this important disease for both organic and conventional growers.  In past evaluations at LIHREC, PMR varieties have exhibited a range in level of suppression from poor to excellent.  It is helpful for growers to know how well new varieties perform in terms of ability to suppress a disease, in order to select a fungicide program to achieve desired level of control, and also to know yield and fruit quality compared to standard varieties to determine whether a new variety is suitable for their operation.  The goal of this project was to evaluate three new acorn squash varieties: Starry Night, Sugar Bush and Tiptop.  Fruit of Starry Night have a unique, pixelated color pattern; all other varieties produce typical dark green fruit.  Two adjacent experiments were conducted, one was treated weekly with PM fungicides starting at first detection of symptoms to evaluate the PMR varieties as part of an integrated management program.

Procedures:

Experiments were conducted at the Long Island Horticultural Research and Extension Center (LIHREC) in Riverhead, NY, in a field with Haven loam soil.  Controlled-release fertilizer (N-P-K, 19-10-9) at 525 lb/A (101 lb/A N) was broadcast over the bed area and incorporated on 22 Jun.  Beds were formed with drip tape and covered with black plastic mulch also on 23 Jun. A waterwheel transplanter was used to make planting holes in the beds and apply starter fertilizer.  Acorn squash were sown in trays in a greenhouse on 4 Jun, then seedlings were transplanted by hand into the holes on 25 Jun. Plants that died were replaced on 29 Jun, 7 Jul and 13 Jul. To separate plots and provide a source of inoculum, there were two powdery mildew-susceptible summer squash plants (cv. Slick Pik) between plots.

Plots were three 10-ft rows spaced 68 in. apart with 12 plants per plot at 2-ft spacing. There were two adjacent experiments. In one experiment, PM was not managed while in the second experiment, it was treated with fungicides to evaluate the PMR varieties as part of an integrated management program: Procure and Vivando applied in rotation weekly. A randomized complete block design with four replications was used.

PM severity on upper and lower leaf surfaces was rated in each plot on 22 and 29 Jul; 3, 10-11, 17-18 and 24-25 Aug; and 1, 8, 15, 22 and 29 Sep. Area Under Disease Progress Curve (AUDPC) values were calculated from 11 Aug through 22 Sep to obtain a summation measurement of PM severity. Fruit were harvested when mature.  Fruit quality evaluations were conducted with CCE and LIHREC staff plus Cornell Gardeners.  Squash were baked and tasted plain without any seasonings or butter.

Results:

Powdery mildew was first observed in acorn squash experiments on 3 Aug in one plot of a susceptible variety; no symptoms were seen the previous 2 weeks.  In the experiment with plants not treated with fungicides for PM, all resistant varieties were significantly less severely affected by PM than the susceptible variety at most assessments (Table 1). A significant difference among resistant varieties was only detected with one assessment.  The fungicide program provided excellent control of PM on upper leaf surfaces (Table 2). Control was considered only good on lower surfaces because starting with the 8 Sep assessment, average severity on lower leaf surfaces was at least 11% for all varieties (8 and 15 Sep data not shown in table).

Control of PM is likely the reason defoliation was substantially less in the fungicide-treated plots than the non-treated (Tables 1 and 2).  Maintaining leaves until acorn squash fruit are mature is important for fruit flavor and minimizing sunscald.  There was indication of better control achieved using both management practices (fungicides applied to resistant variety) based on fungicide-treated resistant varieties having numerically less severe PM than the fungicide-treated susceptible variety for most assessments (Table 2).

There were no significant differences among varieties in average fruit weight (1.6 – 1.8 lb/fruit), number marketable fruit per plant (data not shown) or total number fruit.  Fruit of Starry Night had significantly higher sugar content than all other varieties except Sugar Bush, while Autumn Delight had the lowest Brix values (Table 2).  Starry Night received the highest average rating for taste and texture while Sugar Bush received the highest average rating for shape and external appearance (Table 3).  Fruit of all varieties received both positive and negative comments (Table 4).

Lists and tables of resistant vegetable varieties that include seed companies marketing them and whether organic seed is available, as well as variety resistance to disorders, are available at the Cornell Vegetables website.  Also at that site: Information about powdery mildew and its management.

Acknowledgments: This project was supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch under NYC-153409 and the Friends of Long Island Horticulture Grant Program. 

Margaret Tuttle McGrath
Plant Pathology and Plant-Microbe Biology Section, SIPS, Cornell University
Long Island Horticultural Research and Extension Center
3059 Sound Avenue, Riverhead, NY 11901
mtm3@cornell.edu

Tables

Table 1. Severity of powdery mildew on susceptible and resistant varieties of acorn squash that were not treated with fungicides for this disease, followed by defoliation ratings.

Table 2. Severity of powdery mildew on susceptible and resistant varieties of acorn squash that were fungicide treated for this disease, followed by defoliation and yield data. 

Table 3. Fruit of susceptible and resistant varieties of acorn squash evaluated on a 1 (poor) to 5 (excellent) scale for several parameters by CCE and LIHREC staff plus Cornell Gardeners.  Values are average rating, overall average, and percentage of raters who indicated they would buy the variety. 

Table 4. Comments about fruit from evaluators, followed by number of raters who made the same comment if more than one (in parenthesis).

Images

Below, photographs taken on 9 September showing healthier leaf canopy due to managing powdery mildew with fungicides and/or resistant variety.