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Guidelines  for  Managing  Cucurbit  Powdery Mildew  with  Fungicides  in  2005

Margaret Tuttle McGrath

Department of Plant Pathology, Cornell University,

Long Island Horticultural Research and Extension Center,

3059 Sound Avenue, Riverhead, NY 11901; mtm3@cornell.edu

For current managment recommendations see Powdery Mildew Management Guidelines for 2010

 

Figure 1. Because powdery mildew colonies typically are larger with denser sporulation on lower leaf surfaces than on upper surfaces, control on lower surfaces (left section of this leaf which is folded over) is very important.
For current managment recommendations see Powdery Mildew Management Guidelines for 2010

Guidelines  for  Managing  Cucurbit  Powdery Mildew  with  Fungicides  in  2005

Application of fungicides continues to be the principal practice for managing powdery mildew in cucurbit crops, but successful control is challenged by development of resistance to key fungicides.  While there are varieties with genetic resistant to this disease, an integrated program is recommended to reduce selection pressure for pathogen strains able to overcome the genetic resistance in the plant as well as fungicide resistance. Powdery mildew is the most common disease of cucurbit crops occurring every year throughout the US.  The pathogen develops best on the lower surface (underside) of leaves (Figure 1), thus a successful management program necessitates controlling the pathogen on the lower as well as the upper surface to avoid premature death of leaves.  It is difficult to directly deliver fungicide to the lower surface, even with new nozzle types and air assist sprayers.  Consequently, an important component of fungicide programs has been fungicides able to move to the lower leaf surface. Most of these fungicides are systemic (e.g. Topsin M, Nova) or have translaminar activity (e.g. Flint, Amistar, Cabrio).  Some, notably the new fungicide Quintec, have high volatility enabling redistribution from upper to lower leaf surfaces.

Unfortunately, these fungicides effective on lower leaf surfaces have been prone to resistance development due to their single-site mode of action. Additionally, the cucurbit powdery mildew fungus has demonstrated ability to evolve new strains resistant to these fungicides. Presence of resistant strains has been associated with control failure.  With some fungicides, including MBC (methyl benzimidazole carbamate) fungicides aka benzimidazoles (e.g. Topsin M) and QoI (quinone outside inhibiting) fungicides aka strobilurins (e.g. Flint, Cabrio, Amistar), this change renders the pathogen strain completely resistant to the fungicide (qualitative resistance). With other fungicides, including the DMI (demethylation inhibiting) fungicides (Bayleton and Nova), pathogen strains exhibit a range in fungicide sensitivity depending on the number of genetic changes they possess that affect the fungicide’s ability to function (quantitative resistance). Level of DMI resistance currently is such that the first DMI fungicide registered, Bayleton (triadimefon), is no longer effective and thus no longer registered, while Nova (myclobutanil) is still effective at high rates.

Managing resistance is therefore an important consideration when selecting a fungicide program for powdery mildew of cucurbits. It is important to realize that the primary goal of resistance management is to delay its development rather than just to manage resistant strains after they have been selected; therefore, a management program needs to be implemented before resistance develops to have the greatest benefit.  An important component of a resistance management program is to reduce the need for fungicides that have high risk for developing resistance.  This can be accomplished for cucurbit powdery mildew by growing varieties with genetic resistant to mildew and separating main-season from spring plantings.  Several resistant varieties of pumpkin and winter squash have recently been commercialized. Using an integrated program with fungicides applied to resistant varieties is highly recommended for managing both fungicide resistance and new pathogenic races able to overcome varieties with genetic resistance.

The general fungicide program being recommended for resistance management consists of an alternation among effective high-risk fungicides in at least 2 chemical classes
Figure 2.  Powdery mildew is being effectively controlled with fungicides on upper but not lower leaf surfaces in this pumpkin field.
applied on a 7-day schedule, with a protectant fungicide included in every application.  The first application needs to be made as soon as possible after reaching the threshold of 1 leaf with symptoms out of 50 older leaves examined. It is especially important to examine the lower leaf surface when scouting. Another important component of resistance management is assessing control by routinely monitoring mildew severity on the lower surface of leaves.  When control is poor on lower surfaces but good on upper surfaces (Figure 2), stop using fungicides at risk for resistance (eg Pristine and Nova) and contact local extension specialists promptly so that samples of the fungus can be collected for testing.  A high-risk fungicide should be tank-mixed with a protectant fungicide because the latter has multi-site activity and thus low resistance risk.  It will control pathogen strains resistant to the high-risk fungicide.  For more information on protectants, see the article entitled "Protectant fungicides for managing powdery mildew in  cucurbits:  how  do  they  stack  up?" at:

http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Cuc_Cntct_Fcides.htm

Current recommendation for 2005 is to apply Pristine plus a protectant fungicide on odd weeks and the DMI fungicide Nova plus protectant fungicide on even weeks for at least 4 weeks in main season crops like pumpkin, then apply protectant fungicides alone for the remainder of the season unless another disease controllable with QoI fungicides has developed. A 14-day spray schedule is suitable with powdery mildew resistant varieties.  Pristine is recommended because it contains a new systemic active ingredient, boscalid, as well as the QoI pyraclostrobin.  QoI resistance appears to be sufficiently common and widespread that control with QoI fungicides (Amistar, Cabrio, Flint) likely will be compromised wherever these are used. Pristine is labeled at 12.5–18.5 oz/A, which will cost about $28.49-$42.16/A.  It has a season maximum of 4 applications. Nova should be applied at the highest labeled rate to ensure effective control and avoid selection of isolates able to tolerate lower rates. Boscalid is a group 7 fungicide, pyraclostrobin is in group 11, and myclobutanil, the active ingredient in Nova, is in group 3.

A specific exemption under Section 18 of FIFRA has been requested to permit use of Quintec in 2005. Quintec, which contains quinoxyfen, an active ingredient in a new chemical class, was issued a crisis exemption for use on non-edible-peel cucurbits in 2004 in NY due to poor control in 2003 associated with QoI resistance. Quintec could be applied at 4 fl. oz/A (about $16.40/A), a maximum of 3 times in alternation with other fungicides (eg Pristine and Nova) to melons, winter squash, gourds, and pumpkin.  If the Section 18 request is granted, Quintec will be recommended in a 3-way alternation with Pristine and Nova.  Quintec will be a valuable addition to the fungicide program especially considering resistance occurs to the QoI component of Pristine and to low concentrations of Nova.  Like DMI fungicides, Quintec is only effective for powdery mildew.

NYS DEC has denied registration of the new DMI Procure on cucurbits citing the need for addition studies to document that the active ingredient (triflumizole) does not significantly impact groundwater when used according to labeled rates.  Recent field dissipation study indicates that the half-life is much longer than that described from a previous study (more than 100 days versus 18 days).  Also, mobility of triflumizole is a concern for soils in a sand textural class with low organic materials.  The registrant has concluded that revenues from potential sales of Procure in NY will not be enough to cover the cost of the studies needed, thus it does not plan to conduct this work.

As a result of QoI and DMI resistance, the cost of controlling powdery mildew with fungicides has increased greatly.  If the cucurbit powdery mildew fungus was fully sensitive to fungicides and there was no risk of resistance, it would be possible to manage powdery mildew and obtain control of several other diseases by alternating between Nova at the lowest labeled rate of 2.5 oz/A, costing about $9.86/A, and Cabrio at 12-14 oz/A, costing about $19.13-$22.31.  Due to resistance, Nova needs to be applied at 5 oz/A ($19.71/A).  Also Pristine ($28.49-$42.16) needs to be substituted for Cabrio.  In addition, a protectant fungicide is needed at each application to further manage resistance and minimize the impact on control should resistance develop.  A highly effective inexpensive product is sulfur* (Microthiol Disperss at 4 lb/A costs about $2.64/A).  The cost of a fungicide program for controlling powdery mildew if resistance was not an issue is $97/A for 3 sprays of Nova (2.5 oz/A) alternated with 3 of Cabrio (14 oz/A).  And the cost of a similar program for managing powdery mildew where QoI and DMI resistance occurs is $141/A for 3 sprays of Nova (5 oz/A) alternated with 3 of Pristine (14.5 oz/A), plus Microthiol Disperss (4 lb/A) included with each application. *note that sulfur can be phytotoxic on melons, especially if applied when temperatures are hot.

Monitoring Resistance in 2003 and 2004. Fungicide resistance was monitored on Long Island during the 2003 and 2004 growing season using a seedling bioassay.  Squash seedlings were treated with fungicide, then placed with non-treated seedlings in a production field with powdery mildew for up to a day (Figures 3-4).  The seedlings were kept in a greenhouse until symptoms appeared.  For the first assay, seedlings were placed during late July in spring plantings of squash and in pumpkin fields that had not been sprayed with high-risk fungicides.  Additional assays were conducted in pumpkin fields in August and September.

2003 Results.  Resistance to QoIs was detected in 1 of 5 fields where the bioassay was conducted in July. Moderate resistance to DMIs was common in all fields.  A second assay was conducted at the end of August in pumpkin fields where high-risk fungicides had been used.  DMI and protectant fungicides had been used in all 7 fields; QoIs also were used in 6 fields. Resistance to QoIs was detected in all fields.  Several QoI-treated seedlings had as much powdery mildew as non-treated plants indicating a very high frequency of resistance.  Seedlings treated with a DMI fungicide had less powdery mildew, indicating a lower percentage of isolates with reduced sensitivity to this fungicide group. Powdery mildew did not appear to be suppressed very well on the underside of leaves in these fields.  Resistant strains were also detected during a third assay conducted in organic and conventionally-managed fields that had not been sprayed with high-risk fungicides. For more information on this work, see the article entitled "Occurrence of strobilurin resistance and impact on managing powdery mildew of cucurbits" at:

http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Cuc_Strob.htm

2004 Results. Resistance to QoIs was found, often at a high level, in all 8 fields where the bioassay was conducted in July. Moderate resistance to DMIs and resistance to benzimidazoles was also common (Table 1). (Figures 3-6)

Evaluation of Fungicide Programs in 2004. Flint (2 oz/A) tank-mixed with sulfur (4 lb/A Microthiol Disperss) and applied in alternation with Procure (8 oz/A) plus
Figure 3.  Seedlings for a powdery mildew fungicide sensitivity assay in an early spring-planted summer squash crop on 29 July 2004. Figure 4.  Seedlings for a powdery mildew fungicide sensitivity assay in a main-season pumpkin crop on 29 July 2004

sulfur on a weekly schedule effectively controlled powdery mildew on pumpkin through the end of August.  By the September 13 assessment, control on the underside of leaves had dropped to 69% compared to 93% where a similar program was used with Quintec substituted for Flint and 98% where Quintec was applied alone on the same dates. Quintec will never be recommended as a stand-alone product because it is at-risk for resistance development.  It was tested alone to assess its efficacy.  Control at that time was 87% with Pristine (14.5 oz/A) plus sulfur applied in alternation with Nova (5 oz/A) plus sulfur.  This degree of control was not significantly different from that obtained with Procure and Quintec.  In contrast, control observed in several commercial pumpkin fields on Long Island in 2004 was very poor on lower leaf surfaces despite the fact growers had used Nova as well as Quintec under a 24c crisis exemption (Figure 2).  Since control was excellent on upper leaf surfaces, application timing was good.  Poor control was at least partly due to a delay in the first application of Quintec as product was not available when powdery mildew was first observed. Excellent control was reported elsewhere in NY.  Powdery mildew cannot be effectively controlled when fungicide applications are started after the disease is established.  Based on AUDPC values, which summarize severity over the entire season, control on the underside of leaves was significantly more effective when block applications of Procure and Quintec were used than when a strict alternation was used (88% versus 98%).  The block application schedule was Procure + sulfur (week 1, 4, 5), Quintec + S (week 2,3), and sulfur alone (week 6).

A highly effective protectant fungicide is an important element of a fungicide program for managing fungicide resistance and powdery mildew.  Protectant fungicides are needed

Figure 5.  Leaves on 7 August 2004 from seedlings that were in an early spring-planted summer squash crop 9 days earlier.  Based on powdery mildew severity on leaves treated with the QoI Flint, the DMI Nova, and the MBC Topsin M relative to the non-treated control leaf, strains of the pathogen resistant to these 3 fungicide groups were present at a high frequency in the field the seedlings were placed in. Figure 6.  Leaves on 7 August 2004 from seedlings that were in a main-season pumpkin crop 9 days earlier. Based on powdery mildew severity on leaves treated with the QoI Flint, the DMI Nova, and the MBC Topsin M relative to the non-treated control leaf, strains of the pathogen resistant to these 3 fungicide groups were present at a moderate to high frequency in the field the seedlings were placed in.

because they control pathogen strains resistant to high-risk fungicides and, since they have low resistance risk, they will reduce the impact on control when resistance develops to other fungicides being used.  There are numerous products available to choose from.  Six products with a plant oil as the active ingredient and a hydrogen dioxide product were evaluated in another experiment conducted in 2004. GC-3 organic fungicide (cottonseed oil, corn oil, and garlic extract), Organocide (sesame oil) and Eco E-Rase (jojoba oil) were as effective as Microthiol Disperss, JMS Stylet-oil (mineral oil), and Bravo Ultrex.  The copper hydroxide product Champion provided excellent control in another experiment with pumpkin conducted in 2004.

Please Note: The specific directions on fungicide labels must be adhered to -- they supersede these recommendations, if there is a conflict.  Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.

Table 1. Proportion of cucurbit powdery mildew fungal population estimated to be moderately insensitive to DMI fungicides (eg Nova) and proportion resistant to QoI fungicides (eg Flint) or MBC fungicides (eg Topsin M) based on results from a fungicide sensitivity seedling bioassay. 

 

Resistant  isolates (%)  on  July  29,  2004

Site

Crop

QoI

DMI

QoI + DMI

MBC

1

Pumpkin

39

20

1

36

2

Summer squash

91

57

59

91

2

Pumpkin

56

26

34

84

3

Summer squash

88

78

64

75

4

Summer squash

87

84

53

90

5

Pumpkin

66

15

24

73

6

Pumpkin

32

31

50

31

 

For current managment recommendations see Powdery Mildew Management Guidelines for 2010

 

 

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