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How Long Are Spores In Soil With Early Blight

Early blight usually affects tomato (Lycopersicon esculentum) and potato (Solanum tuberosum) plants, spreading quickly enough to infect your entire crop before it's time to harvest, although many infected plants continue to produce fruit. If you had a problem with the fungal disease last year, your plants are not safe this year. The spores survive more than a single season, meaning they can live to infect new plants by overwintering in the soil.

How Long Are Spores in Soil with Early Blight

Caused by the Alternaria fungus, early blight usually starts as brown dots on the lower leaves of your tomato and potato plants, although it also affects stems and fruit. The brown circles grow concentrically, eventually causing the leaves to yellow or the fruit to rot. Infected leaves eventually fall off, and it the plant loses too many leaves, it' might die or produce less fruit.

When the fungus produces spores, they spread primarily through wind and rain; the rain both washes them off leaves and splashes them upward from the soil onto lower leaves. Spores that land in the soil in fall survive by overwintering in organic matter, such as plant debris. Although they prefer warm, moist weather, spores protected from the worst of the elements -- such as slightly under the soil's surface or under last year's leaf pile -- survive fluctuating temperatures, including being constantly frozen and thawed. When you plant in spring, the spores are ready to attack a new generation of tomatoes and potatoes.

Early blight spores can live longer than one winter in the soil -- one year typically is the minimum. The exact potential lifespan isn't known, but if you plan to help control the disease by rotating crops out of infected areas, keep the areas bare for at least four years, recommends the Colorado State University Extension. Rotation requires a wide degree of separation, and shifting rows a few feet to one side won't save them from traveling spores, but moving your garden to the other side of your yard might.

Regardless of how long the spores have lived in the soil, they have the best chance of infecting your tomatoes and potatoes and reproducing when the right conditions exist. Early blight spores prefer warm weather -- 80 degrees Fahrenheit or warmer. They need moisture to survive, but they require alternating periods of moisture and dryness to properly reproduce. Using drip irrigation rather than overhead watering can help reduce the spread of the disease by keeping the plants as dry as possible.

The fungus spends the winter in infected plant debris in or on the soil where it can survive at least one and perhaps several years. It can also be seed-borne. New spores are produced the following season. The spores are transported by water, wind, insects, other animals including man, and machinery. Once the initial infections have occurred, they become the most important source of new spore production and are responsible for rapid disease spread.

Mulching with shredded tree leaves or straw immediately after transplanting tomato seedlings or planting tomatoes in plastic mulch will help reduce the spread of disease spores. Irrigating the soil and not wetting the foliage will also make a big impact on disease management.

Betweencrops, the early blight fungus can overwinter on potato refuse in the field, insoil, on tubers, and on other solanaceous plants. Infection occurs when sporesof the fungus come in contact with susceptible leaves and sufficient freemoisture is present. Spore germination and infection are favored by warmweather and wet conditions from dew, rain, or sprinkler irrigation. Alternately,wet and dry periods with relatively dry, windy conditions favor spore dispersaland disease spread. Tubers can be infected as they are lifted through the soilat harvest. If sufficient moisture is present, spores germinate and infect thetubers.

It is important to alternate between different chemical families to avoid the development of pathogen insensitivity to particular active ingredients. Some insensitivity to the chemical family 11 has become more common in some areas, so particular care should be taken to rotate these with other chemical families. Also, if insensitivity is already present in a given field population of early blight, fungicides in chemical family 11 will not provide good control.See the Midwest Vegetable Production Guide for current recommendations.

In most production areas, early blight occurs annually to some degree. The severity of early blight is dependent upon the frequency of foliar wetness from rain, dew, or irrigation; the nutritional status of the foliage; and cultivar susceptibility.

The first symptoms of early blight appear as small, circular or irregular, dark-brown to black spots on the older (lower) leaves (Figure 1). These spots enlarge up to 3/8 inch in diameter and gradually may become angular-shaped.

Early blight lesions can be diagnosed in the field easily due to the dark concentric rings alternating with bands of light-tan tissue, giving them a distinctive target spot appearance (Figure 1).

Symptoms of early blight infection on tubers appear as dark and sunken lesions on the surface (Figure 7). Tuber lesions may be circular or irregular in shape (Figure 8) and can be surrounded by a raised dark-brown border.

The underlying tissue is dry with a corky texture and a dark-brown color (Figure 9). Tuber symptoms of early blight infection may become manifest only after months of storage (Figure 10) and can be confused easily with Fusarium dry rot (Figure 11).

The primary infection of potato foliage by A. solani is caused by inoculum provided from other infected hosts or inoculum that overseasoned on infected plant debris. Overwintering spores that serve as the initial inoculum move within and between fields carried by air currents, windblown soil particles, splashing rain, and irrigation water.

Minimum and maximum temperatures for sporulation and infection of potato by A. solani are 41 and 86 F, respectively, with an optimum temperature of 68 F. Following sporulation, spores are disseminated by wind or splashing from rain or irrigation water, and the number of airborne spores peaks in midmorning and declines throughout the afternoon and at night.

Under high humidity and free moisture, and within the range of temperatures, spores landing on leaves of susceptible plants germinate and may penetrate host epidermal cells directly or enter through stromata or wounds. Many cycles of early blight spore production and lesion formation occur within a single growing season.

In addition to survival on infested plant debris, spores and mycelia of A. solani can survive between growing seasons in infected potato tubers and in the overwintering debris of other susceptible solanaceous crops and weeds, including tomatoes and hairy nightshade. Additionally, spores can survive freezing temperatures on or just below the soil surface.

During harvest, tubers often are contaminated with A. solani spores that accumulated on the soil surface during the growing season or were dislodged from desiccated vines. Germinated spores penetrate the tuber epidermis through lenticels and through wounds on the tuber surface caused by mechanical injury.

Tuber infection is most common in potato cultivars that are highly susceptible to skinning, such as red- and white-skinned cultivars. Secondary spread of infection does not occur on stored potato tubers and, unlike late blight tuber lesions, early blight tuber lesions usually do not serve as infection courts for other decay organisms.

Since most commercially acceptable potato cultivars are susceptible to early blight, the application of foliar fungicides is the primary management tactic. Mancozeb and chlorothalonil are perhaps the most frequently used protectant fungicides for early blight management but provide insufficient control under high disease pressure. Therefore, the application of locally systemic and translaminar fungicides often is necessary for control at high levels of disease pressure, especially under irrigation.

Recent research conducted at NDSU by Bauske et al. (2018a) determined that fungicide resistance in the early blight pathogen to the Quinone outside Inhibiting (QoI) fungicides such as Quadris and Headline was highly prevalent. In a survey from 2015, the F129L mutation associated with QoI resistance was detected in 100 percent of isolates collected throughout North Dakota and Minnesota (Table 2). Cross resistance has been documented among many chemistries of the QoIs registered for early blight control in potato.

Resistance to the anilino-pyrimidine (AP) fungicide Scala also has been reported in several states, including Minnesota (Fonseka and Gudmestad, 2016). The accumulation of fungicide-resistance mutations in the early blight fungus has been demonstrated to have no impact on the survival and parasitic ability of this pathogen (Bauske and Gudmestad, 2018), suggesting that resistance is stable and permanent in the A. solani population.

To reduce the potential for fungicide resistance development in A. solani, we recommend that fungicides be tank-mixed and alternated with chemistries of different modes of action. This tactic is facilitated by the Fungicide Resistance Action Committee (FRAC) code number that can be found on every EPA label on the container (Table 1). Furthermore, selecting later-maturing cultivars, which are less susceptible to early blight, can reduce the number of fungicide applications needed to achieve adequate control.

Stevenson, W.R., Kirk, W.W., and Atallah, Z.K. 2008. Managing foliar diseases: early blight, late blight, and white mold. Pages 209-212 in: Potato Health Management, 2nd Edition. D.A. Johnson, ed. The American Phytopathological Society, St. Paul, Minn.

During wet or humid conditions, like warm days with significant rainfall or morning dew, the fungus that causes early blight can replicate rapidly. Spores can be produced as quickly as 3-5 days after spots first appear.


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