They say that our crops are exposed to about 240 economically important pests and diseases and Downy mildew happens to be that disease affecting the many varieties of roses grown in Kenya today.

Downy Mildew is one of the major disease challenges to the floriculture industry whose control budget is about 20% of the total pesticide value. It occurs rapidly and the effects on the quality of roses is irreversible and the loss is irreparable.

What is downy mildew?

Downy mildew is a fungal disease that mainly causes destruction of leaves leading to massive defoliation. It’s a disease of moist cloudy conditions which is active only in cool and damp conditions that causes severe and rapid defoliation - can defoliate an entire plant in 2 days. The causal organism being Peronospora sparsa, a fungus that is highly destructive and fast spreading in roses.

Downy mildew affects different parts of the plant such as the leaves, stems, peduncles, calyxes and petals hence affecting young growth which may wilt. The leaf symptoms are Irregular-shaped, purple spots that form on the upper leaf surface while light grey brown spore masses form on the underside of the leaf. The brown spots look like injuries caused by a chemical spray. Older leaves infected first and then progressively younger leaves follow.

The fungus under unfavourable conditions, survives as mycelium or oospores in or on plant parts without showing disease signs but upon favourable conditions, mycelium growth resumes among the plant cells to produce oospores (fungal seeds) within 3 days. Oospores are spread by wind or water and germinate on leaf or other plant surface. Unlike powdery mildew spores, which are spread by the wind, downy mildew is spread by splashing water. Some farms carry over the disease from bud wood taken from infected stock during propagation.

How do you control downy mildew?

That’s the question in some of if not all grower’s mind and the many who are able to control it are able to identify the disease and it’s stages, looking for symptoms of early infection on older leaf tissue before chlorosis develops and identifying the symptoms before the disease reaches flowers, understanding the biology, the predisposing factors, monitoring the disease’s progress which helps in predetermining the disease onset and coming up with an appropriate chemical intervention. Controlling the disease can be summarized into;

Field and Crop Hygiene: This involves maintaining greenhouse cleanliness, removing infected plant tissues & fallen plant debris well away from your production area since spores may survive and be viable on dried fallen leaves for as long as one month. Cultural Practices which may include opening up the crop by pruning, improving greenhouse air ventilation by opening up vents especially at sunset when the greenhouse air is warm and moist while outside air is cool and drier. It is critical to keep relative humidity below 85% to decrease sporulation on infected plants and stop germination of spores on healthy plants. Humidity should not remain above 85% for more than 3 hours

Chemical Intervention: This is the last but not least method of controlling downy mildew. Crop protection products safeguard harvest yields, and thereby ensure that the flowers are of high quality. Many products give excellent control when used as preventive treatments rather than curative treatments. Growers should categorize the fungicides into Penetrant, Protectant, Post-infection, eradicant, Immune system enhancement etc. Another notable factor is the Spectrum of activity, Resistance concerns and IPM management coupled with crop safety.

There is need to position the chemical so as to interrupt the disease progress at the right point as well as the correct application which is critical.

BASF knows the threat of downy mildew and is constantly innovating new solutions that can stand the test of twenty first century grower demands and challenges. BASF has introduced Orvego® 525SC whose activity is superior in Downy mildew control.

Orvego® 525SC is a versatile element for your downy mildew spray program that maximizes the marketable yields of your crops – for all markets worldwide. It has dimethomorph and Initium™ that stops the flow of energy in the fungus like a valve stops the flow of fuel in a pipe as well as no cross resistance observed with other chemical classes. Initium is an innovation breakthrough by BASF and forms a very vital tool for resistance management as it’s in a class of its own.

The parting shot is that the damage caused by Downy mildew is more or less dependent on the skill and experience of the grower who takes customer satisfaction a notch higher.

Joseph Maina is the BASF Ornamentals Manager - Kenya