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GUIDELINES FOR MANAGING CUCURBIT
POWDERY MILDEW ORGANICALLY
Margaret Tuttle McGrath
Department of Plant Pathology, Cornell University
Long Island Horticultural Research and Extension Center; 3059 Sound Avenue
Riverhead, NY 11901; [email protected]; vegetablemdonline.ppath.cornell.edu
Powdery mildew is the most common disease of cucurbit crops occurring every year throughout
the US. The characteristic white, powdery fungal growth is readily recognizable on leaves and
vines. This growth is mostly spores that are easily dispersed by wind. Symptoms can be
difficult to see on watermelon, however, as spores are produced less abundantly than on other
cucurbits.
Uncontrolled powdery mildew indirectly affects yield because infected leaves usually wither and
die. Premature loss of leaves can result in reduced market quality because fruit become
sunburnt, have poor color, or have low sugar content due to ripening prematurely or
incompletely. Fruit with low sugars have poor flavor and poor storability. Handles on pumpkin
fruit may be shriveled or rotten in addition to fruit being paler orange. Size and/or number of
fruit can be reduced in summer squash and in other crops when powdery mildew is severe.
Severe disease can also lead to imperfections on fruit rind such as speckling and oedema. In
addition, powdery mildew infection predisposes plants to other diseases, in particular, gummy
stem blight (aka black rot).
Powdery mildew can be managed with resistant varieties and regular foliar applications of
disease control products and fungicides*. It is not possible to escape infection because the
pathogen produces many wind-dispersed spores, cucurbit crops are grown widely, and conditions
often are favorable for this disease. The powdery mildew fungus tolerates a wide range of
temperatures below about 100 F and it does not need a period of free moisture on leaves to
infect, in contrast with other foliar fungal pathogens. Rain is actually unfavorable.
There are now varieties of most cucurbits types with genetic resistance to powdery mildew
(Table 1). Unfortunately it can be challenging to find organically-produced or non-pesticidetreated seed. Some seed companies are willing to obtain non-treated seed when available for
large orders or regular customers.
Races of the powdery mildew fungus have been differentiated on cantaloupe. With race-specific
resistance it is important to know what races of the pathogen are likely to be present before
selecting a variety. If a race is present that the variety does not have resistance for it may become
as severely infected as a susceptible variety. Both race 1 and 2 have been present recently on
Long Island, NY. Resistant varieties of cantaloupe and also cucumber can exhibit high levels of
resistance with very few symptoms developing.
Disease onset is delayed and/or development slowed in resistant squash and pumpkin. However,
these varieties can become severely infected by the end of the season. Varieties with resistance
from both parents (homozygous) usually develop less powdery mildew than those with resistance
from one parent (heterozygous). In catalogues these are often described as resistant and tolerant,
respectively.
The powdery mildew fungus has the potential to evolve such that it can overcome genetic
resistance. Selection pressure for new strains will increase as resistant varieties are grown more
extensively; therefore, an integrated program with a few foliar applications is recommended to
thwart selection of new races of the pathogen able to overcome the resistant variety as well as to
improve control in squash and pumpkin. Watermelon was considered naturally immune, but
recently crops have been affected, often severely, from Texas to New England, indicating
development of a new pathogen strain.
The fungus causing powdery mildew is sensitive to many types of chemicals that are approved
for organic production. These include oil (mineral and botanical types), sulfur, copper,
potassium bicarbonate, and biofungicides (Table 2). Efficacy and cost vary widely (Table 3).
Note that copper is the only product labeled for several other diseases. For effective control,
applications need to be started early in disease development and repeated every 7 to 10 days (14
days with resistant varieties). Established powdery mildew cannot be controlled, not even with
systemic conventional fungicides.
Routine scouting is needed to ensure applications are started very early in powdery mildew
development. Plants are susceptible to powdery mildew when in their reproductive phase and at
any age when grown under greenhouse conditions. These are critical times to examine plants.
Start in the field when first fruit start to enlarge. The scouting protocol entails weekly examining
both leaf surfaces of 5 old, crown leaves in at least 10 locations through out a field. Symptoms
develop first on older leaves, often on the underside. It is time to start applications when
powdery mildew is found at a very low level on at least 1 of the 50 leaves. Once symptoms are
easily seen without hunting, potential to effectively manage powdery mildew with rescue
treatments is greatly diminished. Symptoms have also been observed on young field-grown
plants that were severely stressed due to delayed transplanting or surrounded by tall weeds;
removing these stresses can halt powdery mildew development thereby avoiding the need to start
applications early
The pathogen develops best on the lower surface (underside) of leaves, thus a successful
management program necessitates controlling the pathogen on the lower as well as the upper
surface to avoid premature death of leaves. Unfortunately there are no products with systemic
activity approved for organic production and it is difficult to directly deliver fungicide to the
lower leaf surface, even with new nozzle types and air assist sprayers. Foliar applications of
sulfur are more effective than most other organic products for powdery mildew on lower surfaces
apparently because when deposited on the upper leaf surface, sulfur can volatilize and be
redistributed to the lower surface. Sulfur is also less expensive than other materials. Like
copper, sulfur is an element and thus cannot be degraded and removed from the environment as
can materials such as oils. Sulfur is a micronutrient for plants. Note that sulfur can be
phytotoxic on melons, especially if applied when temperatures are hot.
In summary, to ensure good yield of high quality fruit of cucurbit crops, manage powdery
mildew by selecting varieties with resistance, examining foliage weekly to detect when this
disease begins to develop, and applying control products to foliage beginning very early in
disease development.
* note that in the US products for which the manufacturer claims disease control must be
registered as fungicides with EPA unless the ingredients are ‘generally recognized as safe’
(GRAS) by EPA.
Table 1. Powdery mildew resistant cucurbit varieties.
Cucumber, slicers: Autograph, Burpless 26, Calypso, Carolina, Cobra , Competitor , Dasher II,
Daytona, Diamonte, Diva, Eureka (F1), Fanfare, General Lee, Genuine (F1), Greensleeves,
Impact , Indio, Indy, Intimidator, Jackson Classic, Lafayette Classic, Lynx, Marketmore 76,
Marketmore 86, Olympian (F1), Panther, Prancer, Slice More (F1), Socrates (F1), Speedway,
SR2389CW, Stonewall, Suyo Long, Sweet Slice, Tasty Bright, Tasty Green, Tasty Jade,
Thunder, Thunderbird, Turbo, Tyria (F1), Ultra Pak
Cucumber, pickles: Alibi, Amour , Calypso, Classy, Cross Country, Eclipse, Eureka, Fancipak M
, Feisty, FM5020 , Jackson Classic, Jackson Supreme, Lafayette, H-19 Little Leaf, Moxie,
Napoleon , Patton, Pioneer, Salty, Sassy, SMR58, Spear It, Spunky, Timor, Vlasstar, Wellington,
Wisconsin, Zapata
Muskmelon: Ambrosia, Aphrodite, Asian, Athena, Burpee Hybrid, Bush Star, Classic, Cresent
Moon, Earligold , Eclipse, Harper Hybrid, Minerva, Odyssey, Primo, Pulsar, Saticoy, Sugar
Bowl, Sweet’n Early
Melon speciality: Arava, Bolero, Cream de Menthe, Crete, Dorado, Edonis, Gallicum, Golden
Beauty, Honey Brew, Marygold, Minnesota Midget, Rayyan, Savor, Sweetie #6 (Butterscotch)
Pumpkin: Aladdin, Bumpkin, Gold Dust, Gladiator, Gold Boullion, Magician, Pure Gold,
Harvest Princess, Super Herc, Harvest Time, Magic Lantern, Hobbit, Merlin, Iron Man, Golden
Condor, One Too Many, Scarecrow, Touch of Autumn
Summer squash, yellow: General Patton, Patriot II, Sunglo, and Sunray
Summer squash, zucchini: Payroll, Sebring
Winter squash, acorn: Autumn Delight, Royal Ace, Table Star, Taybelle PM
Winter squash, butternut: Bugle, RBX 3106
Winter squash, specialty: Bush Delicata, Celebration, Cornell's Bush Delicata, Harlequin, Metro
PMR, Sweet Lightning, TipTop PMR
Table 2. Products evaluated for managing powdery mildew organically, active ingredient,
whether exempt from tolerance or EPA registration as a fungicide, labeled diseases and insect
pests, and company. Note that a few products are not yet OMRI approved or listed.
1. Bugitol. 0.42% Capsaicin and related Capsaicinoids, 3.7% Allyl Isothiocyanate. Not
OMRI approved yet. Kills and repels numerous insects. Champon Millennium
Chemicals
2. Eco E-RASE. 97.50% Jojoba Oil. OMRI listed. Exempt from tolerance. Controls
powdery mildew and white fly. IJO Products
3. GC-3 Organic fungicide. 30% Cottonseed Oil, 30% Corn oil, 23% Garlic extract. Exempt
from EPA registration. OMRI listed. Controls powdery mildew. JH Biotech, Inc.
4. Organocide. Active ingredients=5% sesame oil; inerts = 92% fish oil + 3% emulsifiers.
Exempt from EPA registration. OMRI listed. Organic Laboratories, Inc.
5. Sporan. 17.6% Rosemary Oil; Other Ingredient = 82.4% Wintergreen Oil. Exempt from
EPA registration. OMRI listed. Labeled for several diseases. EcoSmart Technologies.
6. Trilogy. 70% clarified hydrophopic extract of neem oil. OMRI listed. Certis USA
L.L.C.
7. JMS Stylet-oil, organic formulation. 97.1% paraffinic oil. OMRI listed. JMS Flower
Farms, Inc.
8. Serenade. Bacillus subtilis. Labeled for several diseases. OMRI listed. AgraQuest.
9. Sonata. Bacillus pumilus. Labeled for several diseases. OMRI listed. AgraQuest.
10. OxiDate. 27% hydrogen dioxide. Labeled for several diseases. Will be OMRI listed
soon. StorOx, another formulation, is OMRI listed. BioSafe Systems.
11. Kaligreen. 82% Potassium bicarbonate. OMRI listed. Controls powdery mildew. AgBio,
Inc
12. MilStop. 85% Potassium bicarbonate. OMRI listed. Controls powdery mildew and other
diseases. BioWorks Inc.
13. Champion. Copper hydroxide. OMRI listed. NuFarm Americas Inc.
14. Microthiol Disperss. 80% Sulfur. Will be OMRI listed soon; other sulfur products
available. Cerexagri.
15. Nutrol. Monopotassium phosphate (0-50-32). Not OMRI listed. LidoChem, Inc.
Table 3. Efficacy of products for managing powdery mildew organically, rate, cost, and percent
control achieved in efficacy experiments conducted at Cornell’s research facility on Long Island.
The conventional fungicides Bravo, Quadris, Quintec and Procure were included for comparison.
Treatments and rate *
OxiDate (IPM) 40 fl oz/100 gal
OxiDate (curative) 128 fl oz
Sporan 1.5 qt/A
Trilogy 1%
Bugitol 96 fl oz/100 gal
Eco E-RASE 1%
Organocide 2 oz/gal
GC-3 Organic fungicide 1%
JMS Stylet-oil 5 qt/100 gal
Microthiol Disperss (S) 4 lb/A
Bravo Ultrex 2.7 lb/A
Quintec + S; Procure + S
Application
Cost
($/A)
6.41
20.50
29.25
11.75
28.56
30.00
46.88
23.75
7.50
2.64
13.77
Powdery Mildew Control (%)
Upper leaf surface
8/23
9/16
95 bc
13 a **
33 ab
98 c
41 ab
99 c
63 cd
96 bc
62 cd
98 c
76 cd
97 bc
85 cde
96 bc
83 cde
97 bc
77 cd
100 c
92 def
97 bc
76 cd
100 c
99 f
Lower leaf surface
8/23
9/16
69 bc
7 ab
21 abc
74 bc
18 abc
83 bcd
21 abc
80 bcd
14 abc
87 cd
28 abc
69 bc
36 bc
80 bcd
42 c
81 bcd
30 abc
95 cd
21 abc
71 bc
18 abc
97 cd
82 d
* 8/6-7 (8/11 for Bugitol), 8/12, 8/19, 8/25, 9/1, 9/6, and 9/12-13 (7X). OxiDate curative
started with 3 consecutive applications on 8/25, 8/26, and 8/27 when powdery mildew
reached a level of being easily seen but not severe (average severity on older leaves was
1% on upper surfaces and 2% on lower surfaces).
** ‘a’ indicates no control; severity not different from non-treated. Treatments with the same
letter in a column are not significantly different.
Treatments and rate *
Champion WP 2 lb/A
Sonata AS 2 qt/A
Sonata + Champion
Compost tea
Quintec + S; Procure + Bravo
Applic
-ation
Cost
($/A)
$5.70
Powdery Mildew Control (%)
Upper leaf surface
8/23
9/16
95 bc
13 a **
33 ab
33 ab
100 c
99 f
Lower leaf surface
8/23
9/16
69 bc
7 ab
21 abc
21 abc
97 cd
82 d
* 7/27, 8/4, 8/11, 8/18, 8/24, 8/30, and 9/6 (7X). Compost tea was applied undiluted with NuFilm-P (6 oz/A) at 85 gal/A and 40 psi 9 times from 16 Jul to 8 Sep. Other treatments
applied at 100 psi.
** ‘a’ indicates no control; severity not different from non-treated. Treatments with the same
letter in a column are not significantly different.
Powdery Mildew Control (%)
Upper leaf surface
Lower leaf surface
Fungicide and rate/A
1997 1998 1999 2000 2001 1997 1998 1999 2000
Serenade 6 lb
36 b
Prudent Plus 1.4 qt
40 b
Milsana 1%
45 b 85 b 59 c
21 ab 26 b
Armicarb 100 4 lb
23 a 62 c
23 b
Kaligreen 2.2-4 lb
65 ab
62 c 45 bc
Nutrol 20 lb
60 ab 66 cd
68 c 29 b 24 b
Kocide 2000 2-2.25 lb 98 f-h 72 de 64 b
84 d 60 c-e 20 b 26 ab
JMS Stylet-oil 1.5%
93 e
Microthiol Disperss 4 lb 99 g-j 96 i
69 b
93 e 72 d-f 63 efg 31 ab
Bravo Ultrex 2.7 lb
97 cd 98 e
14 ab
Quadris; Nova + Bravo
93 i 100 d 100 e 95 e
58 ef 84 de 98 fg
2001
8 abc
17 bcd
10 a-d
16 bcd
5 ab
21 d
19 cd
55 f
53 f
37 e
72 g
** ‘a’ indicates no control; severity not different from non-treated. Treatments with the same
letter in a column are not significantly different.