Downy mildew in roses is caused by an obligate biotrophic oomycete in the family peronosporacea namely Peronospora sparsa. As the scientific name indicates, the production of spores is sparse and therefore this disease is difficult to diagnose and control. It attacks all types of roses both in the greenhouse, nursery and the landscape from potted miniatures to landscape-size roses and even shrub roses. It causes destruction of leaves, stems, and flowers of the infected plant. The pathogen produces zoospores that have flagella they use to “swim” to ideal infection sites. That is why wet plant surfaces make the disease much more prevalent.

The fungus overwinters in or on plant parts as a vegetative mycelium or in fallen plant debris as hardy Oospores. The pathogen has short development cycles under optimum conditions, produces high quantities of spores and causes an irreversible damage to the crop.

Reproduction:
They reproduce by forming sporangiophores and sporangia that develops and grow out of the undersurfaces of the infected leaves. Downy mildew (Oomycete fungi) are referred to as a high risk pathogens because of the following factors;

• Oomycetes fungi are able to spread in an explosive manner under favorable conditions.
• Short development cycle (8-10 days under optimum conditions)
• High potential for reproduction (high quantities of spores)
• Wide propagation by water and wind
• Damage is not reversible: The damaged tissues die in general leading quickly to substantial losses at harvest
• High genetic variability: Rapid appearance of strains less sensitive to specifically acting fungicides possible.

Predisposing factors for the Diseases Development.
The optimal conditions for the appearance of Downy Mildew are constant high humidity (RH 85-100%) low night temperatures and moisture on the leaves. The optimal temperatures for spore germination is between 10°C and 18°C no germination take place at temperatures below 5°C and the spores are killed at temperatures above 30°C.

The spores germinate within 4 hours in water, enter the leaves, and reproduce in three days. Spores survive on dried fallen leaves for as long as one month. Wet leaves and high humidity will trigger sporulation overnight. When the sun comes up, leaves start to dry, and spores are released. Most spores spread by wind and infect new leaves before noon. Six hours of constant leaf wetness is enough for spores of downy mildew fungi. These can be summarised as:

1. Susceptible host or varieties.
2. Virulent pathogen that has ability to infect.
3. Conducive environmental conditions
4. Period that the conducive environmental condition prevails.

Source of infection:

1. Infected leaves, sepals, flower buds and stems produce oospores
2. Dormant mycelia in stems
3. Sporangia production
4. Dried fallen leaves

For many Downy mildew species, the sporangia are produced in the evening and released into the air in the morning. They are then spread within the green houses via moist air currents, fingers, contaminated tools or equipments and clothings.

The factors which favour the downy mildew and its quick spread are:

• Type of greenhouse
• Crop type and density
• Drip irrigation
• Nutrition status
• Human activity; pruning, scouting, spraying, harvesting etc.

Symptoms
Downy mildew symptoms on rose include lesions on the leaves, stems, and flowers. Leaves have reddish black spots that are angular, tan spots with a very small amount of white crystalline sporulation on leaf underside. The leaves develop purplish red to dark brown irregular spots or blotches, which might be mistaken for spray burns or possibly black spot. Advanced infections will have yellowing of leaves with brown necrotic areas and noticeable leaf abscission. Severe defoliation may occur as a result of infection. The symptoms are suggestive of chemical injury or possibly nutrient stress. The downy mildew fungus forms a downy mass of spores on the underside of leaf lesions.

Management:

1.Cultural:

• Eliminate conditions that favor leaf wetness early in the day since this condition is critical for downy mildew development.
• The spore release and movement commonly occurs in the morning.
• Adequately spacing plants to promote air circulation and allow wet plants to dry rapidly
• Prune plants to keep them aerated and reduce the RH to <85
• Avoid wide temperature fluctuations that may lead to condensation (diurnal range).
• Greenhouse curtains management
• Destroy rose debris from previous crops — spores can overwinter in leaves and canes, then the downy mildew can attack new plants.
• Even though fans might move spores, you should use them along with venting to reduce humidity and leaf wetness
• Hungry plants are more susceptible to downy mildew. Maintain a balanced fertility program to protect your crops
• Use resistant varieties for low maintenance plantings

2. Sanitation:

• Remove plant debris.
• Remove infected plants as soon as you discover them since the pathogen can reproduce several times a season, potentially causing a devastating outbreak.

3. Chemical control:
Choosing the most effective fungicides to prevent or eradicate rose downy mildew can be tough. Downy mildew requires a wellmanaged chemical spray program starting early with a rotation of chemicals for prevention. Fungicides for use against downy mildew can be categorized as preventive, early or late curative products.

The disease also overwinters in the crop that was infected in the previous season. The fungus may overwinter in stems as dormant mycelia without oospores as shown alongside. This is the primary inoculum of the disease and upon reaching the favourable conditions, the disease infects new stems. This can be controlled through early drenching of Previcur Energy. The preventive products e.g. Antracol must be applied before an infection period begins. New growth following application will not be protected. Early curative products e.g. Infinito, work against spore germination, sporangia elongation and penetration. Late curative products” e.g. Verita and melody duo, deal with intracellular infection level (by this time symptoms are visible to the eye)

• Early curative products that work against spore germination, sporangia elongation and penetration as illustrated below,
• Late curative products” that deal with intra- cellular infection level (by this time symptoms are visible to the eye) These late curative products deal with stages like Cyst germination, mycelium growth as illustrated below in the circle.

Summary

• Chemical compounds can only prevent spread to other plants
• Damage to the Infected plants cannot be reversed and must partially or completely be removed and destroyed
• Consider mode of action to fit with integrated resistance management (IRM)
• Combine protectant and curative activity