Category Archives: Powdery Mildew

Time to Start Scouting for Powdery Mildew

This is a re-post from 2016 – Late-July to early-August is the key time for powdery mildew management!  With any disease, preventative management provides the best control.

Powdery mildew typically arrives in Southern Ontario in mid-to-late July. Plants are most susceptible to infection during the fruit sizing and development. Poor control results in decreased yield and poor fruit quality at harvest. The threshold for treatment is 1 lesions/50 plants. Optimum powdery mildew control is a combination of variety selection, fungicide timing and fungicide selection.

Powdery Mildew Lesion on the Lower Leaf Surface
Powdery Mildew Lesion on the Lower Leaf Surface

Cheryl Trueman, a vegetable pest management researcher at the University of Guelph’s Ridgetown Campus, has been conducting powdery mildew efficacy trials since 2009. In these trials, several products consistently provided good control of powdery mildew. These products are powdery mildew targeted, and have a single site mode of action. To prevent the development of resistance, it is essential to always rotate between different fungicide groups and/or tank mix with a broad spectrum fungicide.

Powdery Mildew Targeted Fungicides Showing Consistent Control in the Ridgetown Field Trials:

Group 13: Quintec (quinoxyfen)
Quintec was the most consistent powdery mildew product tested in Ridgetown. It provided excellent control in 4/5 years and good control in 1/5 years tested.

Group U8: Vivando (metrafenone)
Vivando provided excellent control in 1/3 years and good control in 2/3 years and tested.

Group 7: Fontelis (penthiopyrad), Aprovia (benzovindiflupyr), Sercadis (fluapyroxad) and Pristine (boscalid/pyraclostrobin).
Fontellis was somewhat less consistent. Control with this produce ranged from excellent to poor, depending on the year. It provided excellent control in 1/5 years, good control in 2/5 years and poor control in 2/5 years. Note: Aprovia, Sercadis and Pristine were not tested in the Ridgetown Campus trials.

Group 3: Inspire (difenoconazole), Proline (prothioconazole) and Quadris Top (azoxystrobin/difenoconazole)
Inspire provided a level of control similar to Fontelis; good control in 3/5 years, and poor control in 2/5 years. Proline and Quadris Top were only tested for one year in the Ridgetown trials, in which they both provided good control.

Fungicides containing chlorothalonil (Bravo ZN and Echo) provided a lower level of powdery mildew control, but are still better than the untreated checks. They also control a broad range of other foliar diseases including scab and alternaria.

Research in Ontario and other jurisdictions indicates that the group 11 (QoI) fungicides, Cabrio (pyraclostrobin) and azoxystrobin (a component of Quadris Top) no longer control powdery mildew. However, they may provide control of other cucurbit diseases such as anthracnose and alternaria.

Additional reading: http://www.omafra.gov.on.ca/IPM/english/cucurbits/diseases-and-disorders/powdery-mildew.html#advanced

Time to Start Scouting for Powdery Mildew

Powdery mildew typically arrives in Southern Ontario in mid-to-late July. Plants are most susceptible to infection during the fruit sizing and development. Poor control results in decreased yield and poor fruit quality at harvest. The threshold for treatment is 1 lesions/50 plants. Optimum powdery mildew control is a combination of variety selection, fungicide timing and fungicide selection.

Powdery Mildew Lesion on the Lower Leaf Surface
Powdery Mildew Lesion on the Lower Leaf Surface

Cheryl Trueman, a vegetable pest management researcher at the University of Guelph’s Ridgetown Campus, has been conducting powdery mildew efficacy trials since 2009. In these trials, several products consistently provided good control of powdery mildew. These products are powdery mildew targeted, and have a single site mode of action. To prevent the development of resistance, it is essential to always rotate between different fungicide groups and/or tank mix with a broad spectrum fungicide.

Powdery Mildew Targeted Fungicides Showing Consistent Control in the Ridgetown Field Trials:

Group 13: Quintec (quinoxyfen)
Quintec was the most consistent powdery mildew product tested in Ridgetown. It provided excellent control in 4/5 years and good control in 1/5 years tested.

Group U8: Vivando (metrafenone)
Vivando provided excellent control in 1/3 years and good control in 2/3 years and tested.

Group 7: Fontelis (penthiopyrad), Aprovia (benzovindiflupyr), Sercadis (fluapyroxad) and Pristine (boscalid/pyraclostrobin).
Fontellis was somewhat less consistent. Control with this produce ranged from excellent to poor, depending on the year. It provided excellent control in 1/5 years, good control in 2/5 years and poor control in 2/5 years. Note: Aprovia, Sercadis and Pristine were not tested in the Ridgetown Campus trials.

Group 3: Inspire (difenoconazole), Proline (prothioconazole) and Quadris Top (azoxystrobin/difenoconazole)
Inspire provided a level of control similar to Fontelis; good control in 3/5 years, and poor control in 2/5 years. Proline and Quadris Top were only tested for one year in the Ridgetown trials, in which they both provided good control.

Fungicides containing chlorothalonil (Bravo ZN and Echo) provided a lower level of powdery mildew control, but are still better than the untreated checks. They also control a broad range of other foliar diseases including scab and alternaria.

Research in Ontario and other jurisdictions indicates that the group 11 (QoI) fungicides, Cabrio (pyraclostrobin) and azoxystrobin (a component of Quadris Top) no longer control powdery mildew. However, they may provide control of other cucurbit diseases such as anthracnose and alternaria.

Additional reading: http://www.omafra.gov.on.ca/IPM/english/cucurbits/diseases-and-disorders/powdery-mildew.html#advanced

Pumpkins and Squash Disease Update: 15-Aug-2105

Downy Mildew – While downy mildew is a common occurrence in cucumbers, it is less so in pumpkins and squash.  However, growing regions across eastern North American are experiencing unusually high pressure from downy mildew in all cucurbit crops this year.  Symptoms and spores were recently identified in a pumpkin field in Norfolk county.  While the disease is typically less aggressive in pumpkins and squash than it is in cucumbers, it is still a good idea to scout fields regularly for symptoms.  If the scouting results show that the levels of disease are increasing, downy mildew targeted sprays may be warranted to keep later-maturing crops healthy.  Keep in mind that fungicides commonly used for powdery mildew control will not control downy mildew.

Powdery Mildew – powdery mildew is active in most pumpkin fields by now.  Fungicides must be applied at the very early stages of this disease in order to be truly effective.  Once symptoms are readily visible on the upper-leaf surface, it is usually too late to really benefit from control measures.  When scouting, look for white, powdery spores on the lower surface of the leaf.  There may be a corresponding light-green or yellow lesion on the top surface.  Symptoms usually first appear on the mid-portion of the plant, or even the petioles.

Powdery Mildew Lesion on the Lower Leaf Surface
Figure 1. Powdery Mildew Lesion on the Lower Leaf Surface

Powdery Mildew in Pumpkins and Squash

There are low levels of powdery mildew showing up in the pumpkin and squash plots on campus this week.  Powdery mildew is the most important disease of pumpkins and squash. Melons and cucumbers are less susceptible, although infections also occur in these crops.

Powdery mildew does not overwinter in Ontario.  It typically appears in late-July each year. While some varieties have a good level of resistance to this disease, many do not. Severe infections result in decreases in yield, sugar content and harvest quality. Control is especially important in Halloween pumpkins where infections cause the stems to decay resulting in poor (or absent) handles.

Many of the products commonly used for downy mildew control in cucumbers do not control powdery mildew.  Despite the similar names, the two diseases are quite different. Each disease has its own set of registered fungicides. An effective fungicide program for pumpkins and squash is very different from one used in cucumber crops. Continue reading Powdery Mildew in Pumpkins and Squash

Powdery Mildew in Cucurbits

Powdery mildew has also arrived in Ontario.

Unlike many fungal pathogens, powdery mildew does not require prolonged leaf-wetness for infection to occur; in fact spores may geminate at humidity levels as low as 20%!  The development of this disease does slow down at temperatures above 30 C.  However, slightly cooler night-time temperatures may result in an unexpected infection period.

Powdery Mildew on Lower Leaf Surface
Powdery Mildew on Lower Leaf Surface

Fungicide efficacy is optimized when they are used preventatively. Scout fields regularly and initiate a spray program no later than the first sign of infection.  Look for small, white powdery lesions on the underside of the leaf surface (Figure 1). Symptoms typically first appear on the leaves in the mid-portion of the plant or on the stems.

It is important to keep all new growth adequately protected.  The leaves are most susceptible to infection 16-23 days after unfolding.  Pumpkins and squash are particularly susceptible to powdery mildew.  Uncontrolled infections result in premature defoliation, smaller fruit sizes and lower sugar levels.