(Phytophthora infestans)
Characteristics
Leaf symptoms:
First symptoms are small irregular shaped, water-soaked lesions often near leaf edges. These quickly develop into large, dark brown to black irregular lesions that are not limited by leaf veins, as is early blight. Late blight lesions don't have the tight target look that lesions of early blight do. When conditions are right, the fungus will sporulate, and a thin, white mildew will occur on the underside of the leaf. At times, these spores will only occur on the margins of the lesions. The lesions can have a very distinct yellow halo around them.
Stem symptoms:
The new US8\A2, which is spreading rapidly in the U.S., is characterized by more stem infections. These infections are characterized by brown lesions on the stems and petioles, and these stem infections will also sporulate if conditions are right. These stem lesions are not slimy like those of aerial blackleg and do not extend into the inner tissue, but this infection can kill the stem or petiole. Rhizoctonia in the perfect stage will produce a thin, white mildew at the base of the stem, but it will not have the brown lesion that late blight has. Sclerotinia could also be confused with late blight. It has a more fluffy, cottony white growth without the brown lesion of late blight and, as sclerotinia progresses, it will form black fungal growths in the stem similar to mouse turds.
Tuber infections:
Tuber infections are characterized by irregular shaped, slightly sunken brown to purplish lesions that can extend into the flesh in a granular tan rot. The margin of the diseased tissue is not distinct. Secondary organisms, such as soft rots, usually finish off the tubers. In contrast, early blight tuber infection is usually characterized by black lesions with distinct borders that do not extend deeply into the tuber flesh. One of the first things that occurs after tuber infection is that the fungus kills the eyes. This makes planting infected seed a risky business (very poor emergence can occur). In a bad foliar infection situation, there can be less of a tuber blight problem since tubers are more mature and can resist infection and late blight dies in the dying foliage.
Favorable conditions:
Late blight likes high moisture conditions at moderate temperatures. Rain, dew, irrigations are all favorable for rapid growth. Late blight can lay somewhat dormant, or just grow very slowly, on plants for some time, waiting for the right conditions to spread rapidly. Conditions must remain wet for a period of 7-10 hours for sporulation to occur, as such sporulation usually occurs after wet nights or periods of rainfall. In the San Luis Valley conditions may be conducive for sporulation with the heavy dew in the early morning. The duration of the morning dew will be a big factor in late blight sporulation, as well as those occasional rainy periods. In other areas late blight problems have been reduced in dryer years. The morning dew will be much more conducive to late blight than sprinkler irrigation. After sporulation, spores can be carried by wind (up to 100 miles, it's said) or water to other plants. The asexual reproduction stage can take as little as 72 hours to complete its cycle to be ready to sporulate again.
Late blight survives and overwinters on living tissue such as cull piles, infected seed, stored potatoes and volunteer potatoes (such as in a rock pile). Early blight, on the other hand, can survive on dead leaves, and vines, in the soil, as well as infected tubers.
Late blight infects young, healthy tissue, especially if it's high in nitrogen. It is a disease that strikes actively growing plants and will actually die in senescing tissue. Early blight, on the other hand, is more severe in senescing tissue, such as older leaves, a dying plant, or a plant that has been injured, is insect, disease or drought stressed, or one suffering from poor nutrition.
Hosts:
Potatoes
Tomatoes (although in Mexico, A1 type is reportedly to be mostly found on tomatoes)
Hairy Nightshade
Infection:
Infection occurs in healthy tissue from spores in air or water. Infection rates are low for each spore but, since there are so many spores, the disease can spread rapidly if conditions are right. Tuber infection occurs when spores are washed down to tubers. It cannot get into mature, intact tuber skin, but can enter through immature tubers, lenticles, eyes and wounds. Cross tuber infection is fairly difficult to accomplish, but can happen. Spores have a flagellum so they can swim down and infect a plant indirectly. Sexual reproduction is possible, but has not been documented yet in North America.
Virulence:
Late blight is more aggressive than early blight. The new A2 strain may be more aggressive than the old A1 strain of late blight-some A1 types are very aggressive. A2 is probably a more severe problem because it is resistant to a very good tool for controlling the old A1 strain--Ridomil.
Control Measures
-PLANT LATE BLIGHT FREE SEED. This will result in late blight in your field, and seed infected with late blight usually doesn't emerge so stand problems will result. One of the first thing late blight does when it infects tubers is kill all the eyes.
-Don't cut your seed if possible. This has been shown to be an excellent way to spread late bight.
-Mancozeb and Tops Mz have been shown to help prevent some late blight spread in seed tubers, but it is not curative.
-Make sure you get the first fungicide on before rows close, which is already common practice for early blight control programs.
-Follow a normal early blight prevention program with protectant fungicides, such as coppers, EBDC's, Bravo or Terranil and Supertin. This will also give you protection against late blight, you just need a lot for fungicides to control late blight. . These fungicides are also the primary control methods for controlling late blight, so you aren't without protection against late blight if you are treating for early blight. Fungicide intervals should be 7 to 10 days. If the weather is very conducive to late blight (high humidity, rain, lots of dew) or late blight is found in your field or area, this interval may have to be shortened to 5 days.
- There are late blight forcasting models that can predict when late blight may get aggressive. You can save a lot on early fungicides using these models as well as know better when to get serious about fungicide applications. One model is included in the University of Wisconsin's WISDOM program. Colorado State University has an interactive version of this Blitecast model that uses Colorado weather information. Click here to go to that site.
-Scout your fields for late blight -- especially in low areas, wet areas, and right around the pivot where it stays wetter. Also, scout more after wet periods.
-Don't over water or over-fertilize your fields - this just puts you more at risk of losing control if a late blight infection occurs. Frequent irrigations could also encourage the spread of late blight.
-If the first span is over-watering potatoes, which is common, undersize the first nozzles or take out every other nozzle. This small area isn't worth the risk of losing the whole field. Also, avoid another sprinkler double watering your field.
-If it's in your field in a small area, killing the infected area with sulfuric acid or just pulling up the infected plants may control the infection if it hasn't already spread to other parts.
-Protectants that are used in early blight infection are the primary control method. However, there are some differences. Since late blight primarily infects young, growing tissue and stems, it is more important to go to a more frequent interval to keep new tissue protected and to not leave any unprotected areas as fungicide washes off. Good coverage is very important, even on the stems which are easily infected by A2 late blight. Chemigation with a slow sprinkler is not a good way to apply fungicides. A fast sprinkler applying low amounts of water (0.20" or less) can be an effective way to apply fungicides since it wets the entire canopy. Ground-rigs are probably the best way to apply fungicides for late blight control if one can get them consistently. Staying on a schedule is very important, and waiting for an airplane or ground-rig can have disastrous consequences Aerial applications of fungicides usually have low volumes applied and missed areas by planes have resulted in late blight strips in a field. A fungicide interval of 5-10 days should work well, depending on conditions. Weather models have been used successfully in other areas to predict fungicide applications, and should also work well here, since we don't get much rainfall. An incremental fungicide rate program is recommended with an increase in rates as the season progresses. Care must be take to not exceed maximum fungicide application amounts before the season is over. Ridomil alone will not control A2 late blight, but is packaged with fungicides such as Bravo, copper and Mancozeb, that will control A2 late blight. Ridomil may still be useful for pink rot control.
-There are 3 semi-systemics, Curzate, Tattoo and Acrobat, for late blight control. These don't work much better than most protectants and some are very expensive. They still have a place in a late blight control rotation program (probably not much over once per season for one of them). They probably wouldn't be the best to apply as soon as late blight is found somewhere in the Valley. They are useful if conditions get bad or if late blight infection is advancing in your field such as blight lesions are larger that 1 cm in diameter. Fungicide rotation is still important. Remember, it is better to prevent late blight infection, but it can be controlled in many situations if infection occurs.
-This fungicide program can be very expensive, especially if you use expensive fungicides, fungicide combinations and pay for custom application. Remember, in many University studies have shown little or no differences between fungicides for late blight control, just that its important to apply something often and with a good application. For more information on fungicide costs and sample fungicide schedules and budgets click here.
-All these extra fungicides will have other effects on our crops. Using too much Bravo can lead to more white mold problems at times. Using too much EBDC's has lead to increased aphid problems as natural beneficial fungi that kill aphids is eliminated.
-A balanced fertility program is important - do not keep plants too high in nitrogen, especially late season.
-Cull piles of infected tubers are important sources of future infections. These infected culls should not be piled up so that late blight can survive all winter. Spread these tubers less than 6" deep out on a field so the winter can kill late blight in these tubers. Rock piles can also be a source of volunteer potatoes which can carry late blight from one season to another. These should be sprayed with Round-up if they occur. If a field is totally lost, the best thing is to dig the tubers up and leave them on the surface of the ground so that the winter can kill the late blight.
-Don't over-water your fields to avoid any wet spots for late blight to start and spread.
-Harvest should not occur in a late blight infected field until vines are completely dead (this will kill the late blight in the foliage) and tubers have a mature skin. Spores can live 7 days or so on the soil surface, so have everything dead for at least 10 - 14 days before harvest to reduce tuber infection risk. Sulfuric acid is the best vine kill for this purpose. If the field has vigorous vines it may be necessary to beat the vines before sulfuric acid as in the case often with Nuggets. Also, a light irrigation right after sulfuric acid application can increase its effectiveness at killing the vines. Even if there are no signs of late blight on the tubers, this is no assurance they won't break down later in storage or that any seed saved won't be infected. Rolling the hills before harvest may seal them and help prevent late blight tuber infection. Not harvesting in wet conditions will also reduce late blight tuber infection, as well as decreasing skinning and bruising at harvest.
-Adequate air (maybe continuous) and cool storage temperatures (38o F) are important for preventing late blight and secondary organisms from melting down the tubers in storage. If there is more than 5% tuber infection visible at harvest, the crop probably cannot be stored and should be sold immediately. Late blight itself doesn't usually cause serious storage problems, but it opens up the tuber for soft rot infection. Late blight management in storage is mostly soft rot management. Running the storage for a few days without humidity with continuous air is sometimes recommended if a late blight problem is suspected, but no humidity at first can lead to serious shrink losses so use this management technique sparingly. If there is no tuber infection visible at harvest and if late blight tuber infection is going to show up, it usually appears in the first 1 to 1 ½ months of storage. There should not be free water or excess humidity in the storage if late blight is a potential or actual problem.
Late Blight on foliage Late blight on a stem
Late Blight Links
Michigan State Late Blight Laboratory
Note: This information should only be used as a guide. Adjustments for local conditions must always be made.