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Alternaria species exist in soils around the world, and it is considered to be one of the most aggressive soil-borne pathogens known to man. Reported losses in the published literature range from 5 to 40% in potato and 35 to 78% in tomato. It is also known to show harmful effects on animals and humans with more than 70 toxins produced by the mycotoxin of Alternaria found in a wide range of foods and animal feeds.

For all its importance, comparatively little is known about Alternaria solani – early blight as it is also known. As a plant pathogen Alternaria has been the subject of academic research for more than a century, yet relatively few papers have been published. The focus of these has been screening methods and genetics for resistant varieties with a leaning towards tomato.

While our understanding of Alternaria may not be as great as with late blight (Phytophthora infestans), it is sufficient to give growers and their advisers the information needed to develop a management plan. Below are some of the more important findings that will help growers protect crops against infection.

1. Spore dispersal

The dispersal range for A. solani is limited to within the proximity of the field. In contrast, A. alternata, which is a secondary threat that enters the plant via earlier damage and is associated with brown spot, can spread significantly further due to a greater number of spores (up to seven times) that are also smaller. This suggests that new infections of A. solani come from within the field while sources surrounding the field are the cause of A. alternata infections. 

2. Plant debris

Infected plant material on the soil surface can harbour disease until the following season. A. solani can overwinter without a host in a broad temperature range: -3 to 21oC. The fungus can survive on debris for more than eight months.

3. Resistance to fungicides

The mutation conferring reduced sensitivity to substances belonging to the Quinone outside Inhibitors (QoI) group, namely pyraclostrobin, azoxystrobin and famoxadone, dominates across Europe and the USA. Isolates with reduced sensitivity to SDHI active substances, namely boscalid, have also been found to be widespread. European studies reveal that upwards of 40% of isolates received for testing have one of the SDHI mutations conferring reduced sensitivity.  Several studies over the past decade have investigated the extent of any cross resistance between boscalid and fluopyram as both are SDHI substances. None has been detected and fluopyram continues to give good intrinsic control with a narrow range of sensitivity values as measured by the industry standard EC50 test. 

4. Future resistance to fungicides

The strong performance of fluopyram should not negate the need to practice good resistance management. Research published by a consortium of European academics led by the Technical University of Munich (TUM) in June 2021 found that A. solani isolates resistant to QoI and SDHI substances was not the result of a single development that then spread across the continent, but the consequence of resistance evolving multiple times independently.  This “not one person to blame” assessment suggests that while the status of fungicide resistant A. solani isolates in Great Britain is not well understood, it should not be assumed that the genotypes less sensitive to pyraclostrobin, azoxystrobin, famoxadone and, more recently, boscalid, are not present. Even if they weren’t introduced on imported material, they may have developed here independently.

5. Practical crop protection

Plant resistance is of limited value given the evolutionary potential of the pathogen. Varieties with improved resistance mechanisms would need to be introduced on a regular basis to stay ahead of pathogen development. Several biological control agents have shown promise, but further investigations are needed before conclusive evidence of their potential can be decided. In the meantime, good control of sources of inoculum, such as Black nightshade (Solanum nigrum) and use of fungicides, such as Caligula (fluopyram + prothioconazole) which combines multiple modes of action for strong performance, should continue to be the basis of crop protection.

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