{"id":8447,"date":"2026-09-21T17:04:52","date_gmt":"2026-09-21T15:04:52","guid":{"rendered":"https:\/\/www.quelagrow.com\/blog\/best-fungicide-potato-late-blight-guide\/"},"modified":"2026-09-21T17:04:52","modified_gmt":"2026-09-21T15:04:52","slug":"best-fungicide-potato-late-blight-guide","status":"publish","type":"post","link":"https:\/\/www.quelagrow.com\/en\/blog\/best-fungicide-potato-late-blight-guide\/","title":{"rendered":"Best Fungicide for Potato Late Blight: Guide and Solution"},"content":{"rendered":"<div style=\"background: #f8fafc; padding: 25px; border-radius: 10px; border-left: 6px solid #1e6ea7; margin: 25px 0; box-shadow: 0 2px 8px rgba(0,0,0,0.1);\">\n<h2 style=\"color: #1e6ea7; font-size: 24px; margin-bottom: 15px; font-family: 'Inter', 'Segoe UI', sans-serif; font-weight: 600;\">\u2705 KEY TAKEAWAYS \u2013 Best fungicide for potato late blight<\/h2>\n<ul style=\"list-style: none; padding-left: 0; color: #2d3748; font-size: 17px; line-height: 1.7; font-family: 'Inter', 'Segoe UI', sans-serif;\">\n<li style=\"margin-bottom: 12px;\">\u2022 <strong style=\"color: #1e6ea7;\">What it is:<\/strong> <strong>potato late blight<\/strong> is caused by the oomycete <strong>Phytophthora infestans<\/strong>, a pathogen distinct from true fungi even though it&#8217;s managed within the same bioprotection strategy.<\/li>\n<li style=\"margin-bottom: 12px;\">\u2022 <strong style=\"color: #1e6ea7;\">Two fronts:<\/strong> it attacks both foliage and tubers, with consequences in the field and during post-harvest storage.<\/li>\n<li style=\"margin-bottom: 12px;\">\u2022 <strong style=\"color: #1e6ea7;\">Risk conditions:<\/strong> relative humidity above 90% and mild temperatures (50-68\u00b0F) sustained for several hours \u2014 watch closely after intermittent rain or persistent dew.<\/li>\n<li style=\"margin-bottom: 12px;\">\u2022 <strong style=\"color: #1e6ea7;\">Solution:<\/strong> <strong>QUELAFERT\u00ae FOS-K<\/strong>, a potassium phosphite formulation with reinforcing action specifically against Oomycetes, applicable both foliarly and via root uptake.<\/li>\n<li style=\"margin-bottom: 12px;\">\u2022 <strong style=\"color: #1e6ea7;\">Sustainable approach:<\/strong> potassium phosphite strengthens crop defenses with a low-residue profile, fitting a bioprotection strategy that&#8217;s more environmentally friendly than relying solely on synthetic fungicides.<\/li>\n<\/ul>\n<\/div>\n<p>&nbsp;<\/p>\n<h2>What is potato late blight and how to recognize it<\/h2>\n<p><strong>Potato late blight<\/strong> is, historically, one of the most impactful documented agricultural diseases: it was responsible for the 19th-century Irish famine, an episode that illustrates just how completely this pathogen can wipe out a harvest when conditions favor it and no action is taken in time. It&#8217;s caused by <em>Phytophthora infestans<\/em>, an oomycete \u2014 not a true fungus, though it shares much of its agronomic management with them \u2014 capable of spreading extremely fast once it finds enough moisture on plant tissue.<\/p>\n<p>Recognizing late blight early is the difference between an effective preventive intervention and a hard-to-reverse crop loss. If your issue isn&#8217;t a fungal disease but an insect pest instead, check out our guide on <a href=\"https:\/\/www.quelagrow.com\/en\/blog\/biological-pest-control\/\">biological pest control<\/a>.<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone size-full\" src=\"https:\/\/www.quelagrow.com\/wp-content\/uploads\/hoja-patata-afectada-por-mildiu.jpg\" alt=\"potato late blight on leaf\" width=\"1381\" height=\"752\" \/><\/p>\n<p>On <strong>leaves<\/strong>, the first warning is usually the appearance of olive-brown lesions with irregular, oily-looking margins, typically starting at the leaflet edges or tip. When ambient humidity is high \u2014 especially in the morning after overnight dew \u2014 a <strong>white, cottony mycelium<\/strong> can be seen on the underside, formed by the pathogen&#8217;s sporulation structures. If humid conditions persist, lesions spread rapidly and the leaf tissue becomes brittle, helping spread spores to neighboring plants via wind and rain splash.<\/p>\n<p>On <strong>tubers<\/strong>, the damage is even more economically decisive, since it directly affects the marketable part of the harvest. The infection shows as a <strong>reddish-brown rot<\/strong> that penetrates from the skin inward, in irregular, diffuse-edged patches. This damage isn&#8217;t always visible at harvest: the pathogen can keep progressing in storage, so an apparently healthy lot can develop rot weeks later.<\/p>\n<p>&nbsp;<\/p>\n<h2>Causes and conditions that favor late blight<\/h2>\n<p><em>Phytophthora infestans<\/em> depends on a very specific combination of environmental conditions to complete its infection cycle: <strong>high relative humidity<\/strong>, generally above 90%, and <strong>mild temperatures<\/strong>, roughly 50-68\u00b0F, sustained for several hours in a row. This is why late blight alerts tend to coincide with spells of intermittent rain, persistent fog, or heavy dew.<\/p>\n<p>The pathogen also persists remarkably well from one season to the next. It survives in infected crop debris left in the field, and especially in <strong>contaminated seed tubers<\/strong>, which act as the main entry route into otherwise healthy fields. If your field is also carrying a deeper soil-borne problem, check our guide on <a href=\"https:\/\/www.quelagrow.com\/en\/blog\/soil-disinfection-agriculture-without-harming-microbiome\/\">soil disinfection without harming the microbiome<\/a>. Once established, secondary spore dispersal by wind and rain splash can spread the disease rapidly within and beyond the field.<\/p>\n<p>&nbsp;<\/p>\n<h2>Best fungicide for potato late blight: QUELAFERT\u00ae FOS-K<\/h2>\n<p>Against a pathogen with this spreading capacity, management can&#8217;t be limited to reacting once symptoms are visible: prevention and reinforcing the plant&#8217;s natural defenses play a decisive role. This is where <strong>QUELAFERT\u00ae FOS-K<\/strong> fits in as a specific tool within our <a href=\"https:\/\/www.quelagrow.com\/en\/bioprotectors\/fungicides\/\">fungicides<\/a> range, the bioprotection line that at Quelagrow, as a <a href=\"https:\/\/www.quelagrow.com\/en\/\">fertilizer manufacturer<\/a> and biocontrol producer, we develop against diseases like late blight.<\/p>\n<p>&nbsp;<\/p>\n<p><a href=\"https:\/\/www.quelagrow.com\/en\/bioprotectores\/fungicidas\/potassium-phosphite\/\"><img decoding=\"async\" class=\"alignnone size-medium\" src=\"https:\/\/www.quelagrow.com\/wp-content\/uploads\/quelafert-fosk-300x300.jpg\" alt=\"QUELAFERT\u00ae FOS-K - Potassium phosphite\" width=\"300\" height=\"300\" \/><\/a><\/p>\n<p><a href=\"https:\/\/www.quelagrow.com\/en\/bioprotectores\/fungicidas\/potassium-phosphite\/\">\u203a View product<\/a><\/p>\n<p>&nbsp;<\/p>\n<h3>How QUELAFERT\u00ae FOS-K works against late blight and Oomycetes<\/h3>\n<p>At Quelagrow we formulate <strong>QUELAFERT\u00ae FOS-K<\/strong> as a product very rich in phosphorus \u2014 supplied as phosphite, not phosphate \u2014 and potassium, two elements that, in this specific chemical form, activate the plant&#8217;s own defense mechanisms beyond their usual nutritional role. Once absorbed, phosphite moves easily through the whole vascular system, reaching both above- and below-ground tissue, giving it systemic action.<\/p>\n<p>QUELAFERT\u00ae FOS-K <strong>boosts the plant&#8217;s defenses against fungal-type attacks<\/strong>, with action particularly marked against the <strong>Oomycete<\/strong> group \u2014 precisely the group <em>Phytophthora infestans<\/em> belongs to \u2014 and also against <em>Phytophthora parasitica<\/em>, responsible in other crops for collar rot, damping-off and gummosis.<\/p>\n<p>This matters more than ever in a context where reducing reliance on synthetic fungicides is increasingly valued. Phosphites, as a group of substances, are characterized by low-to-no phytotoxicity and a much lower residue profile than many conventional fungicides, making them a coherent piece of <strong>sustainable agriculture<\/strong> and integrated pest management strategies.<\/p>\n<h4>How to apply QUELAFERT\u00ae FOS-K<\/h4>\n<p><em>(Pending exact dose and application schedule once the client confirms the technical data sheet.)<\/em> The product supports both <strong>foliar and root application<\/strong>, allowing the treatment to be adapted to the crop&#8217;s growth stage.<\/p>\n<table style=\"width: 100%; border-collapse: collapse; margin: 25px 0; font-family: 'Inter', 'Segoe UI', sans-serif; background-color: #ffffff; box-shadow: 0 1px 3px rgba(0,0,0,0.1); border-radius: 8px; overflow: hidden;\">\n<thead style=\"background-color: #1e6ea7; color: white;\">\n<tr>\n<th style=\"padding: 15px; text-align: left; border: 1px solid #e2e8f0;\">AFFECTED PART<\/th>\n<th style=\"padding: 15px; text-align: left; border: 1px solid #e2e8f0;\">MAIN SYMPTOM<\/th>\n<th style=\"padding: 15px; text-align: left; border: 1px solid #e2e8f0;\">RECOMMENDED TIMING<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr style=\"background-color: #f8fafc;\">\n<td style=\"padding: 12px 15px; border: 1px solid #e2e8f0; font-weight: 600;\">Leaf<\/td>\n<td style=\"padding: 12px 15px; border: 1px solid #e2e8f0;\">Oily olive-brown spots + white mycelium underneath<\/td>\n<td style=\"padding: 12px 15px; border: 1px solid #e2e8f0;\">Preventive, ahead of prolonged high-humidity spells<\/td>\n<\/tr>\n<tr>\n<td style=\"padding: 12px 15px; border: 1px solid #e2e8f0; font-weight: 600;\">Tuber<\/td>\n<td style=\"padding: 12px 15px; border: 1px solid #e2e8f0;\">Diffuse-edged brown rot from skin inward<\/td>\n<td style=\"padding: 12px 15px; border: 1px solid #e2e8f0;\">Preventive, during tuber bulking<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>&nbsp;<\/p>\n<h2>Prevention best practices<\/h2>\n<p>Phytofortifier treatment pays off much more when combined with agronomic management that reduces disease pressure from the start. These are the cultural practices with the most real impact:<\/p>\n<ul>\n<li><strong>Crop rotation:<\/strong> avoid planting potatoes or other nightshades consecutively on the same field.<\/li>\n<li><strong>Plant spacing:<\/strong> excessive density restricts air circulation and keeps leaves damp for longer.<\/li>\n<li><strong>Proper drainage:<\/strong> prevents waterlogging that favors both the pathogen and plant stress.<\/li>\n<\/ul>\n<p>At Quelagrow we see these cultural practices as inseparable from <strong>integral bioprotection<\/strong>. Discover the rest of our <a href=\"https:\/\/www.quelagrow.com\/en\/bioprotectors\/\">bioprotection solutions<\/a>.<\/p>\n<p>&nbsp;<\/p>\n<p><img decoding=\"async\" class=\"alignnone size-full\" src=\"https:\/\/www.quelagrow.com\/wp-content\/uploads\/patata-afectada-por-mildiu.jpg\" alt=\"potato tuber infected with late blight\" width=\"1264\" height=\"843\" \/><\/p>\n<p>&nbsp;<\/p>\n<h2>Frequently asked questions about the best fungicide for potato late blight<\/h2>\n<h3>What is the best fungicide for potato late blight?<\/h3>\n<p>There&#8217;s no single product that solves late blight on its own: effective management combines cultural prevention with phytofortifiers like <strong>QUELAFERT\u00ae FOS-K<\/strong>.<\/p>\n<h3>Does late blight only affect the leaves?<\/h3>\n<p>No. It affects both foliage and tubers, where rot can keep progressing even after harvest.<\/p>\n<h3>Under what conditions does late blight appear?<\/h3>\n<p>Mainly high relative humidity (above 90%) and mild temperatures sustained for several hours, typical of intermittent rain or persistent dew.<\/p>\n<h3>Is potato late blight the same disease that affects tomatoes?<\/h3>\n<p>Yes. Both potatoes and tomatoes are nightshades and both are susceptible to <em>Phytophthora infestans<\/em>. If you grow both nearby, monitor them together.<\/p>\n<h3>Can a plant with advanced late blight be cured, or is it only preventable?<\/h3>\n<p>Necrotic tissue doesn&#8217;t recover. Real management is preventive: reinforce defenses ahead of risk periods and remove heavily affected material promptly.<\/p>\n<h3>How do I tell late blight apart from other leaf spots caused by water stress or nutrient deficiencies?<\/h3>\n<p>The key is the white, cottony mycelium on the underside of the leaf \u2014 neither irrigation issues nor nutrient deficiencies produce that.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Best fungicide for potato late blight: identify leaf and tuber symptoms and protect your crop with QUELAFERT\u00ae FOS-K.<\/p>\n","protected":false},"author":2,"featured_media":8424,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[246],"tags":[],"class_list":["post-8447","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-bioprotectors"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/posts\/8447","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/comments?post=8447"}],"version-history":[{"count":0,"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/posts\/8447\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/media\/8424"}],"wp:attachment":[{"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/media?parent=8447"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/categories?post=8447"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.quelagrow.com\/en\/wp-json\/wp\/v2\/tags?post=8447"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}