With changes in the weather come fungal diseases in roses


Roses are ornamental cut flower plants which fetch high prices at both domestic and export markets.

Fungal diseases have become a threat to the horticulture sub-sector and especially to the rose flower producing farms, says a study by M.E Omunyin and S.K Mutiga.

Although production is done in a greenhouse, weather changes in a year greatly affect the severity of diseases the most prevalent being Powdery mildew, Downy mildew and Botrytis rot.

Botrytis is caused by Botrytis cinerea, Powdery mildew by Spaerotheca pannosa and Downy mildew by Peronospora sparsa. The growth and multiplication of the pathogens are favoured by different environmental conditions hence the difference in severity of the diseases at different times of the year.

The study was conducted from two rose flower growing farms – Equator and Maji Mazuri. Data was collected from the grading hall (stored as “grading reject analysis or faults summary records’). The data showed mean percentage stem loss due to fungal diseases (Downy mildew, Powdery mildew and Botrytis) for two years: 2002 and 2003.

The findings showed that the mean percentage stem loss was high between the months of April to June and also between August and October for each year.

The high loss between April and June was due to Botrytis and Downy mildew since the two diseases are favoured by high relative humidity and low temperatures (rainy season). The other great loss occurs between August and November (when the amount of precipitation is low) with the temperatures being high and low relative humidity.

A paired t-Test was done to test the significant difference in the loss for the monthly loss for the two years and for each farm. A paired t-Test was also used for comparisons of the loss between the farms and for respective years. In each of the above cases, there was a significant difference at S.L of 0.05. The difference could be attributed to variation in management, rose flower varieties or weather.

Extracted from a 2004 study by Omunyin ME and Mutiga SK Department of Horticulture, Moi University, Kenya.