Cool Season Diseases

From BASF: The Chemical Company Website

By Paul Pilon

As many growers begin spring production, they are often surprised by the presence of plant pathogens attacking their crops. Although production temperatures are typically lower than optimal for many diseases to develop during the early spring; many diseases are quite capable of infecting plants with low production temperatures.  It is not uncommon to see some diseases develop at temperatures as low as 50 to 55º F. Below are several diseases growers often experience in the early spring.


Botrytis and Botrytis blight (Botrytis cinerea) is one of the most problematic diseases that attack crops in the early spring.  It can rapidly establish at nearly any production temperature (50 to 84º F) when plants have water on the leaves and the humidity is high.  Botrytis requires dead or damaged plant parts in order for the disease to prosper.  Once it begins to grow on wounded or senescent tissues, it can rapidly spread to healthy leaves and stems.

The damaged tissue first appears as tan to brown water-soaked areas that become gray as they dry out.  Infected flowers usually show small pin-prick type lesions or water-soaked areas, which enlarge rapidly and turn to a brown or black coloration. Gray mold is most commonly identified by the fuzzy gray or brown spore masses that develop on infected plant parts.  Severe infestations can consume the entire flower and cause leaf and stem blights. 

Leaf Spots

Although fungal leaf spots occur most frequently during warm, wet growing conditions, there are a number of pathogens that cause leaf spots under cooler conditions. Leaf spots symptoms vary in size, shape, and color.  Although most leaf spot diseases don’t cause plant mortality, their presence greatly reduces the appearance and marketability of the plants they infect.

Leaf spots are likely to occur during cool growing conditions with high relative humidity and following periods where the foliage remains wet for extended periods. The occurrence of leaf spots can be reduced by avoiding periods of high humidity (>80%), providing adequate air movement and keeping the foliage as dry as possible.

Powdery Mildew

Powdery mildew is typically identified by the presence of small (¼ to ½ inch in diameter) whitish-gray powdery patches of fungal growth on the upper leaf surfaces.  Powdery mildew can remain as numerous small patches or colonies or coalesce and completely cover leaf surfaces.  Some plants exhibit powdery mildew symptoms as a reddish purple discoloration without visible patches of mycelia growth present. Unlike many diseases which require extended periods of free moisture on the leaves for infections to occur, powdery mildew requires extended periods of high relative humidity at night not free moisture for infections to occur.

Root Rots

Root rot pathogens, namely Phytopthora, Pythium, and Thielaviopsis, are also likely to occur with cool growing conditions.  These pathogens are most prevalent on crops or liners that have been carried over during the winter months, but may occur under certain circumstances. Root rot pathogens are generally more difficult to diagnose than foliar diseases.  Symptoms of root rot include chlorosis, stunting, reduced plant vigor and roots that appear brown and mushy. Root rots can largely be avoided by reducing stressful growing conditions such as drought or wet-dry cycles, excessive moisture levels, and high soluble salts.  Thielaviopsis occurs more frequently when the pH is high (>6.0). 

It is not usually necessary to implement preventative programs to prevent these cool season diseases from occurring. To control these diseases, it is best to manage the environment by providing proper plant spacing, adequate air movement, controlling the humidity, and maintaining good fertility and irrigation practices.  Growers should have routine scouting programs to detect the presence of diseases early and to determine if and when control strategies are necessary. 

If chemical controls are necessary here are some of the most effective fungicides (listed alphabetically) for controlling the diseases discussed above.

Botrytis - Daconil, Decree, Iprodione Pro 2 SE, Medallion, and Pageant

Leaf Spots - Daconil, Iprodione Pro 2 SE, Medallion, and Pageant, and Protect                           

Powdery Mildew - Eagle, Milstop, Pageant, Pipron, Strike, and Terraguard

Root Rots -               

        Phytopthora - Aliette, Insignia, Pageant, Stature SC, Subdue Maxx, and Truban

        Pythium - Banrot, Pageant, Subdue Maxx, and Truban  

        Thielaviopsis - Cleary’s 3336/OHP 6672, Medallion, Terraguard                                

Be sure to rotate between chemical classes when applying fungicides to reduce the likelihood of this pathogen from developing resistance to these fungicides.

The mention of specific active ingredients does not constitute an endorsement or recommendation of, or discrimination against similar products not mentioned.