These black spots contain the spores of the fungal disease . Sometimes the margins of the purple-brown spots are yellow . The spots tend to be oval or irregularly shapes and can appear in concentric rings like the rings of a bull ’ s-eye target . When the spots are on leaf veins , the attached leaf will become distorted and yellow overall , before ultimately succumbing to the disease and falling off . Lower , older leaves are generally the first to show signs of this infection . Alternaria spots can appear on the stems as well . These spots are longer than those on the leaf , generally greater than 2.5cm long . They also are often sunken or concave in appearance . When these lesions form at the base of the plant , at the soil line , they can girdle and kill the cannabis plant . These are not the only symptoms that can accompany an infestation of Alternaria , however . Necrotic ( dead ) spots can appear anywhere on an affected plant , not just leaves and stems . They can also appear on parts of the flower , petioles , and bracts . This disease can also cause less obvious symptoms such as slowed or stunted plant growth . It can reduce the host plant ’ s ability to absorb nutrients appropriately . This can of course result in a wide range of other deficiencies and problems , many unseen , which can make positive identification of the disease even more difficult since there can be many non-fungal causes of poor nutrient absorption in a plant .
“ Some estimates place
Alternaria as responsible for as much as 20 percent of all agricultural crop losses due to disease .”
It starts out as a seed-borne fungus . Once a cannabis plant from an infected seed germinates and develops , the disease is transferred into the host plant , and then can infect any nearby plants as well . The disease is slow to develop , and as such affected plants do not often show symptoms until they are mature , even well into the flowering stage . Because the disease can be carried by multiple host species besides cannabis , marijuana plants growing near other crops can be susceptible even if grown from certified clean seed . Common agricultural crops like carrots , potatoes , beans , tomatoes , and herbaceous plants can harbour this disease and spread it to nearby cannabis crops . Most types of Alternaria present as lesions on leaves or stems . There are some forms , however , that can cause cannabis plants to develop sudden root rot which can kill single plants or even entire crops within a matter of hours . Fortunately , this species of the disease is not as common as the version that causes leaf spots .
Symptoms of Alternaria
It can be difficult to diagnose Alternaria on cannabis as its symptoms can look like those of many other diseases . It is most often mistaken for Botrytis , or other common fungal pathogens . Alternaria often first appears as purple to brown leaf spots about 2cm in size . Sometimes there are further black spots on top of these purple-brown spots .
Conditions Favouring Development of Alternaria
Alternaria , like most fungal infections , develops in particularly humid ( above 80 percent ), warm ( over 15 ° C ), and moist conditions . When Alternaria spreads onto a cannabis plant ’ s leaves , what would normally be ideal conditions for plant growth can contribute to the disease ’ s rapid spread and development . Morning dew , coupled with the day ’ s humidity and warmth will help spread the spores of the disease across the plant ’ s leaves and stems . Within half a day , the disease can penetrate the plant , colonize , and start reproducing and spreading to other neighbouring plants , unnoticed to even trained disease scouts . There are other factors besides weather and environmental conditions that can make your cannabis plants particularly prone to the disease ’ s introduction and development . Poor soils and bad nutrient management invite Alternaria as well . Decaying matter and diseased crop debris are also where this disease often develops and breeds before infecting plant hosts . Good crop management practices , including the complete removal or destruction of the prior year ’ s roots , stems , and other spent biomass are critical steps in trying to combat the spread of this common fungal disease .
Treatment
Alternaria is not an easily treated disease of cannabis ( or any other type of crop ). In general , once a plant has it , it is extremely difficult to control and , in most cases , the affected plant ( s ) should be destroyed , as well as any leaves , flowers , or branches that may have fallen off of it . If Alternaria is undetected once cannabis plants have entered the flowering phase , control of the disease is all but impossible . Until that point though , there are a few options .
Maximum Yield 41