Potato Review

www.potatoreview.com POTATO REVIEW SEPTEMBER/OCTOBER 2020 27 ALTERNARIA Alternaria’s annual return Recognise the differences between early blight strains – forewarned is forearmed. R ecent changes to the weather conditions in Britain mean there is never a year without an outbreak of early blight (Alternaria spp), according to Norfolk-based Farmacy agronomist Matt Hardy. ere are two strains of the pathogen that a ect British potato crops: A. alternata which infects crops early in the season, and A. solani, which comes in later. Matt, who looks after potato crops across a large area including Swa ham, Cromer and Norwich, is keen to point out that while initial infections of the two pathogens create similar symptoms, there are some important di erences. With A alternata, dark spots develop on the lower canopy, while for A. solani, they appear on the upper and lower leaves. Of the two, A. solani is more aggressive, developing larger lesions and a faster spread through the crop. It is also more visible as it appears on the upper canopy, and is often found rst on damaged plants, such as those close to tramlines where the tractor has clipped them, on headlands, or because of potato cyst nematodes (PCN). If multiple cycles of spore production occur, the damage can cause premature defoliation and its subsequent impact on yields. Hot weather encourages the disease, particularly when it occurs with alternating short periods of moisture. e high temperatures initiate spore release from crop debris and predisposing plants to infection during the next humid period. Spores and spots When moisture is available, such as after irrigation, the spores germinate and penetrate potato tissue, often through existing wounds. Left unchecked, dark brown or black target spots that appear on leaves can coalesce into lesions and spread, defoliating the canopy. “2020, which has been a di cult year for growers in many ways, has not been the worst year for Alternata, but we have still had to spray for it,” remarks Matt, adding, that so far it has not been a high pressure blight year either. ere are a number of possible reasons why the pathogen is becoming more prevalent, including the tendency to hotter spells of weather in early summer and the reduction in mancozeb use in late blight (Phytophthora infestans) protection programmes. Varietal susceptibility also plays a key role. Matt says: “ e need to grow potato crops – Alternaria (early blight) facts: • Alternaria is a mainly soil-borne fungal pathogen that a ects potato crops. It is a global disease that has been present in UK crops for many years. • Warm and wet periods favour the disease. • It can cause dramatic leaf loss and yield reductions of up to 30%. • e predominant species is Alternaria solani. Another species that a ects potatoes and tends to infect later in the season is Alternaria alternata. It is almost impossible to distinguish the two species without a microscope. • e disease causes lesions on the leaves which often have a target spot appearance of concentric rings. • Leaf tissue dies as the disease spreads. • Symptoms can be mistaken for nutrient de ciency or Verticillium wilt. • e fungus survives between crops on potato or other host debris or directly in the soil as spores. • Periods of wet and dry will favour Alternaria‚Ä’s life cycle and encourage spore germination and release ➜ Symtom of early potato blight disease.