Other root and stem rot diseases in pansy and viola

Several root and stem spot diseases can affect pansies and violas including Pythium and Phytophthora. In many respects the control of these root and stem base pathogens is similar to that for downy mildew as they are related oomycete pathogens.

This information was last updated in 2014.

Root and stem rot diseases

Pythium and Phytophthora species can be troublesome in both pansy and viola crops.

Pythium species tend to be more common, particularly on young seedlings during the propagation stage.

This fungus-like organism infects the root tissues, often when the plant is weakened by some other factor, causing further loss of vigour.

Samples

In the last few years there have been an increasing number of reports of stem base rotting in pansy and viola and, where samples have been investigated in the plant clinic, a Phytophthora species has been isolated consistently.

In this case, the roots have tended to be largely unaffected and this helps differentiate the infection from Pythium species.

In both cases, laboratory examination is necessary for accurate diagnosis.

Spotting the symptoms

Pythium species cause a generalised pale-brown discoloration of the root tissues, often the outer tissues slough off when they are examined.

In the early stages of infection the fine roots become infected and the disease progresses to the main roots reducing root function and overall plant vigour.

In the case of the Phytophthora found on pansy and viola the roots themselves show little or no discoloration especially in the early stages of the disease.

Instead, often the first sign of infection is when the plant collapses due to a severe orange- brown decay of the stem base tissues; by which time it is too late to take intervention action.

Examination at a plant clinic is necessary for accurate diagnosis.

Disease development, spread and survival

In both cases, these pathogens tend to infect the host via motile spores in contaminated water, lateral spread from plant to plant occurs by water splash and in water films at the base of pots in trays.

Similarly both Pythium and Phytophthora species produce thick-walled resting spores either in the root tissues or in the stem base tissues and these can survive adverse climatic conditions to re-infect plants when the conditions are more conducive to infection.

Controlling Pythium and Phytopthora

In many respects the control of these root and stem base pathogens is similar to that for downy mildew as they are related oomycete pathogens.

However, in the case of Pythium and Phytophthora species the target is the root and stem base tissues whereas for downy mildew the primary target is the foliage.

Therefore, the application method is important along with the mode of action of the product itself.

Systemic

Products with systemic activity applied as either a high volume foliar spray or drench to the substrate are likely to be effective against both pathogens.

It is important to check the approval status of each product to ensure the method of application proposed is approved, as this is a statutory condition of approval, whereas the target pathogen is not.

The same guidance in terms of resistance management noted previously also applies with these pathogens.