How to keep control of your greenhouse to combat Downy Mildew on rose stems

Few diseases strike more fear into industrial rose growers across the highland tropics than downy mildew, Peronospora sparsa. The disease can appear quickly when cool temperatures (10-18°C), high relative humidity (> 85%, > 3-4 hrs) and excess moisture on leaves combine particularly on young apical tissue – the weakest point in a plant. There is only one way to combat downy mildew, by taking a zero-tolerance approach across every greenhouse on a farm, and that means taking appropriate prevention methods, combined with early detection and rapid intervention when necessary.

Key indicators of downy mildew

One of the primary signs of downy mildew are purplish red, brown or blackish angular spots on the upper side of a leaf, particularly in areas between major veins. In fact, in humid environments (>85% RH), grey to brownish erect sporangiophores will form on the underside of the leaf and conidia will spread from here by any air currents and water splashes.

Sporulation will not always be visible underneath the leaves, but the risk of this taking hold  when environmental conditions are favourable is high. Purple patches can also be identified on stems, peduncles, floral sepals and petals.

Zeroing in on downy mildew in rose stems

In this blog we will focus on the consequences of downy mildew on rose stems, from identification through to establishing control. This will serve as an excellent best practice guide for the control and management of the disease.

Easily recognisable purple patches of 2 cm or more can be observed on affected stems, often alongside longitudinal cracking of the stem. Under very humid and moist conditions, white mycelial growth may also be observed. Although Aegerter et al. (2002) has shown that symptomatic rose stems are extensively colonized within the stem cortex tissue, researchers have not been able to unanimously confirm whether downy mildew is a systemic disease in the rose plant. Many farm technicians have posited that downy mildew in stem can be dealt with by cutting off the sick part of the harvested stem, in order to meet production expectations. But this comes with long-term issues after harvest.

Short-term gain, long term pain – the false economy of trimming stems

There is significant evidence to show that removing the impacted part of a rose stem only serves to postpone the repercussions of downy mildew. Ing. Roberto Toscano of Hoja Verde Cia. Ltda., Ecuador shared his experiences with downy mildew in stem affecting vase life significantly, even after chopping off the symptomatic part of the stem and rescuing the remaining, although shorter, stem for sale. After two days, the heads of the rescued stems started hanging and did not open up like the heads on the healthy stems harvested within the same bed.

This is why the Scarab Precision monitoring system takes downy mildew in stem very seriously and provides growers with the knowledge and strategy to track the status of downy mildew in the rose crop before harvest. Its findings put farms on high alert, so that they can put in place a proactive strategy to avoid the full impact of downy mildew.

Five steps to establish control

Rose varieties differ greatly in their susceptibility and tolerance to downy mildew, but many of the susceptible varieties are in high demand and fetch a good market price. Here are the five stages to reduce the likelihood or minimize the impact of downy mildew in rose greenhouses:

  1. Prior Prevention
    Prevention by means of climate control is ideal, but for those without such technology, prevention relies on good hygiene and ventilation (except during downy mildew sporulation). Fungicides, such as potassium phosphites, copper phosphites or fosetyl-aluminium, also important in the plant’s early life, help suppress outbreaks of this disease to a certain extent.
  2. Thorough Hygiene
    Hygiene is critical to reduce the presence of downy mildew, as sporangia can remain viable on dried fallen leaves for up to one month and carefully removing the debris around the plants, without stirring up the sporangia into the air, can help reduce the presence of spores.
  3. Adequate Ventilation
    Providing good air movement and keeping the greenhouse well ventilated through appropriate plant spacing, raised roofs, raising curtain walls during times of water condensation in the air, and installing industrial fans (except during downy mildew sporulation) to keep the relative humidity below 85% is key, but difficult during rainy and cool conditions.
  4. Quick Cut and Disposal
    Beyond the preventive measures, when detected, it is vital to spray curatively to avoid the spread of spores and then cut and carefully remove any affected plant parts directly into bags, which should be immediately sealed.
  5. Focus on Fungicides
    For those who don’t have climate management, the control of downy mildew relies heavily on chemicals and to avoid the build-up of resistance, it is critical to apply different active ingredients with different modes of action, as established by the Fungicide Resistance Action Committee (FRAC).

High-volume sprays of fosetyl-aluminium (FRAC 33) or tank mixtures of cymoxanil, mancozeb, and fluazinam (FRAC 27/ FRAC M 03/ FRAC 29) have proven good control. Also, good control has been observed rotating cyazofamid (FRAC 21), dimethomorph (FRAC 40), mancozeb/copper hydroxide (FRAC M 03/FRAC M05), phosphonate fungicides (FRAC 33) and oxathiapiprolin (FRAC 49) according to Salgado-Salazar et al. (2018). Phenylamides (FRAC 4), on the other hand, should be excluded as these are prone to develop resistance.


Active ingredients to be rotated in an effective downy mildew control programme:
Phosphonate fungicides (FRAC 33) work as protectants by boosting the plant’s own natural defences and may be applied in form of Potasium phosphite, Copper phosphite, Calcium phosphite and Magnesium phosphit
Fosetyl-aluminium (FRAC 33) has a systemic mode of action and provides a long-lasting, broad spectrum preventative control of diseases
Cymoxanil (FRAC 27) is a contact and locally systemic fungicide that has curative activity when applied immediately after spore germination
Mancozeb (FRAC M 03) is a protectant fungicide
Fluazinam (FRAC 29) has protective activity with very good residual effect, and limited curative or systemic activity
Cyazofamid (FRAC 21) is a protectant fungicide
Dimethomorph (FRAC 40) is a systemic fungicide with protectant, curative and anti-sporulating activity
Copper hydroxide (FRAC M05) is a protectant fungicide. Caution: There is a high risk of causing phytotoxicity when applied in spray water with a pH of less than 6.5 and/or mixed with fosetyl-aluminium
Oxathiapiprolin (FRAC 49) is a protectant fungicide
Phenylamides (FRAC 4) should be excluded as these are prone to develop resistance


Winning the battle against downy mildew in rose stems

Growers must take an all-encompassing approach to combat downy mildew. Simply removing the affected part of a stem does not solve the issue and can actually do long-term damage to a grower’s reputation through very short vase life.

Controlling downy mildew in rose greenhouses means turning knowledge into action, from scouting to prevention strategies and intervention to regular control. 

By Lisbeth Riis, CEO, Scarab Solutions


Major sources:

Aegerter et al. (2002) Detection and management of Downy mildew in rose rootstock. Plant Disease 86:1363-1368

Salgado-Salazar et al. (2018) Downy Mildew: A serious disease threat to rose health worldwide. Plant Disease, 102:1873-1882

Thanks to:

Ing. Roberto Toscano of Hoja Verde Cia. Ltda., Ecuador, for sharing his experiences.