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Septoria Leaf Spots

August 27, 2013

Septoria is one of the most prolific leaf spot plant pathogens growers face. Once an infection occurs, the leaf spots progress quickly and may cover the majority of the plant in a short period of time. Although Septoria does not result in plant mortality, the presence of leaf spots does greatly decrease the aesthetic value of infected crops.  

Certain crops, such as Rudbeckia, commonly develop leaf spots caused by Septoria during production as well as in the landscape. Septoria diseases infect a wide range of plants including ornamentals, vegetables, and grain crops. There are numerous species of Septoria diseases, each one infecting a narrow range of plant species. For example, Septoria rudbeckiae which infects the perennial plant Rudbeckia is a different species than the Septoria which infects the vegetable tomato (Septoria lycopersici).

Symptoms of infections vary with the species of Septoria causing the infection as well as the plant species being attacked. In most cases, disease symptoms first appear as small, 1/8 to 1/4 inch tan to dark brown spots. The leaf spots usually first appear on the lower leaves. As the disease progresses, it moves up the plant and the leaf spots will appear black; this coloration is caused by the reproductive bodies of the fungal growth. With severe infections, the leaf spots coalesce and can cover much of the leaf surfaces.

Like many plant pathogens, it is often difficult to positively identify Septoria infections using visual observations. It is helpful to submit samples to a diagnostic clinic for proper identification.  

Septoria infections are most prevalent following extended periods of wet, humid weather conditions. Production sites and landscapes with frequent overhead irrigation are also susceptible to infections on susceptible crops. Septoria primarily occurs when the leaves remain wet for long durations and quickly spreads from plant to plant with splashing water.  

The most important aspect of managing Septoria is to avoid extended periods when free water remains on plant surfaces. This can be done by changing from overhead irrigation to drip irrigation where feasible. If the method of irrigation cannot be modified, then use the following strategies.  

Avoid overhead irrigation during cloudy weather, in the early morning or late in the day where the leaves are likely to remain wet for long periods of time; water susceptible crops in the mid morning to early afternoon when the conditions are more conducive to rapid drying of the leaves. Where possible, provide good air circulation by placing crops at wide plant spacing, minimize contact from plant to plant to allow the foliage to dry as quickly as possible. Reduce any water that rests on plant leaves such as dripping from roof condensation or dripping sprinklers.  

If these methods are not providing adequate disease prevention, the application of fungicides is necessary. Fungicides work best when they are applied on a preventative basis or just as infections are beginning to occur. Fungicides containing chlorothalonil, pyraclostrobin + boscalid, fludioxonil, and chlorothalonil + thiophanate methyl are effective at controlling Septoria leaf spots.

Most fungicides need to be applied at 7 to 10 day intervals to effectively prevent Septoria from occurring, particularly in scenarios with frequent overhead irrigation and/or rainfall.  Decrease this interval when conditions are optimal for disease development (during overcast and humid weather conditions). Conversely, the application intervals can be further apart during dry periods or when the irrigation intervals are far apart. Use these fungicides in rotation to manage the potential for these pathogens to build resistance to them.

As with most leaf spot diseases, good cultural practices and irrigation management along with the implementation of preventative or control strategies upon early detection will greatly reduce the severity of Septoria leaf spots.

Paul Pilon
Perennial Solutions Consulting

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