Late Blight Control: An Update
Thomas A. Zitter
Late blight can cause significant losses any season, and with weather conditions very favorable for disease, a review of control options and methods is important. Although we are focusing on potatoes, many of these comments have a direct impact on tomatoes. In 1999, US-8 clonal lineage was responsible for huge potato losses in the Red River Valley, but New York was spared any losses due to our dry conditions. For tomatoes, the US-11 and US-17 clonal lineages are the most important isolates, but again we were spared in New York in 1999. There have been no confirmed reports of late blight in New York on potato at this time (July 26, 2000). Late blight has occurred on tomato in eastern New York, on a commercial farm in Albany Co., and from homeowner tomato gardens in nearby Schenectady Co. Late blight was confirmed in a commercial potato field in Crawford Co., PA (about 50 miles south of Erie). Part of this planting was sprayed to kill the plants, and part is being sprayed aggressively on 5-day schedule.
Scouting for Late Blight. If you are not sure that you can recognize late blight, obtain the Vegetable Crops Fact Sheet 726.20 authored by William Fry in July 1998, with colored pictures of the symptoms of late blight of potatoes and tomatoes. Scout aggressively for signs of late blight, beginning as soon as green tissue emerges. Late blight can occur on stems as well as on leaves. When a canopy develops, look for blight in the lower portions of the plant where the foliage stays wet. Take care not to spread late blight from field to field when scouting. Rubber (or disposable) pants and boots, which can be washed after leaving a field, should be used if disease is present. Inoculum also can be carried from field to field on equipment, but is easily removed by washing with water. Early blight and Botrytis leaf blight are also occurring on potato at this time, so submitting a sample to your county agent would be appropriate for accurate identification.
Fungicide options. We can expect that most inoculum, when it occurs in the state, will be of the new immigrant genotypes which are Ridomil-resistant, and are more aggressive than the older isolates. Therefore, growers should not depend on Ridomil for late blight control. The key for late blight control is to use any of the following materials in a preventative manner, and some are better than others. The protectant fungicides most commonly used are chlorothalonil (Bravo, Equus 720, Echo, and others), mancozeb (Manzate 75DF, Dithane DF, Penncozeb75DF, and Manex II), maneb (Maneb 75DF or Manex), and metiram (Polyram 80DF) in combination with triphenyltin hydroxide (Super Tin 80WP). Bravo Weatherstik has long been an industry standard, and if used at the very early stage of disease development can be quite effective. The newer formulations of Dithane DF Rainshield and Manzate 75DF will provide growers with good control under times of less intense disease pressure. Metiram (Polyram) plus triphenyltin hydroxide (Super Tin) have a place in mid-season sprays when disease pressure from late blight and early blight are less intense, and this combination can be alternated with other fungicides.
When conditions are most favorable for late blight, or if Phytophthora has been reported in the area, we now have three products available for use on potatoes, full registration for Curzate and Quadris, and a section 18 registration for Tattoo C. These products have varying degrees of preventative, curative and locally systemic activity. The key for these products is that they are most effective if used preventatively. Tattoo C is probably the preferred material since it works best under warm weather conditions, has good soil and foliar persistence, and can move upward from soil to new tissue if it gets to the soil in sufficient amounts. Systemic active is best within sprayed leaves, since the product does not move from sprayed tissue back to the stem and then into new tissue. Several years of data from Bill Frys program demonstrated that Tattoo C could suppress leaf lesion expansion, which is consistent with its suppressive effect on an already established epidemic. Tattoo C may be applied by ground or air at a maximum rate of 2.3 pints product (0.9 propamocarb) per acre. A maximum of 11.5 pints product (4.5 lb propamocarb) may be applied per acre per season. A 14-day PHI must be observed.
Curzate 60DF (cymoxanil) works best under cool weather conditions but far less under warm temperatures. The product will persist in the soil and on foliage for a few days, but does not move from the soil into the plant, and like Tattoo C, it does not move from sprayed tissue into the stem and then into new tissue. Curzate is used at the rate of 3.3 oz/A and must be tank mixed with a protectant fungicide such as Bravo or Manzate. Even when tank mixed with a protectant fungicide, it is recommended that the spray schedule using Curzate be switched to a different fungicide program with another mode of action after several applications. Curzate alone (as it should never be used) does not control early blight. Observe a 14 PHI with Curzate.
Quadris (azoxystrobin) is registered for use on both potato and tomato. It will provide control of late blight if used preventatively, but its real strength is providing excellent control of early blight. For potatoes, Quadris is normally used at the rate of 6.2 fl oz/A, but should be increased to 12.4-15.4 fl oz/A if late blight symptoms develop or if conditions favor disease development, and used at 5- to 7-day intervals. For tomato the top rate is 6.2 fl oz/A. For both potato and tomato, Quadris should be used in a strict one to one alternation with a fungicide with a different mode of action such as chlorothalonil or another late blight effective material. The PHI of Quadris on potatoes is 14, but may be applied to tomatoes to the day of harvest (0 PHI).
Fungicide rates. Fungicide labels specify a range of rates to accommodate the particular needs as dictated by weather, crop development, and disease occurrence. Follow manufacturer recommendations.
Late blight forecasts. High relative humidity and leaf wetness (from rainfall, dew, fog, or irrigation) are favorable for late blight. The favorable temperature range is very wide, but the disease proceeds most quickly when average (day and night) temperatures are 59-80 degrees F. The higher the temperature, the more quickly "disease severity values" accumulate.
Disease forecasting systems express the effects of temperature and relative humidity on disease development as "severity values". Local late blight forecasts will be available for many potato production areas. However, accurate weather data are required for accurate disease prediction, and this is best achieved with weather monitoring machines in each field, especially if fields vary greatly in specific location and topography. Unless you know that the forecast information is appropriate for your fields, THE FORECASTS SHOULD ONLY BE USED AS A GENERAL INDICATION OF HOW FAVORABLE WEATHER HAS BEEN FOR LATE BLIGHT.
Application methods. The key word here is coverage. Thick canopies, stem infections, and lower canopy infections, all translate into the need to deliver fungicides in the most effective manner. Air-assist sprayers have definitely been shown to be superior in product delivery, but not every grower has this equipment on his farm. There are still actions that can be taken to maximize coverage and deliver to the lower canopy. Slowing down the tractor speed and adjusting boom height to the appropriate level over the plant canopy, are just two suggestions that should be followed. It might not be a bad idea to enter the field from a different direction each time you spray to provide more adequate coverage throughout the canopy.
Reporting late blight. County agents/specialists and consultants have been asked to report the occurrence of late blight on a county-by-county basis. This is valuable information for tracking the regional occurrence of late blight within the state. Submitting samples to the Plant Disease Diagnostic Clinic at Cornell University in Ithaca, NY 14853 (329 Plant Science Bldg., Attn. Karen Snover) is also important for confirmation. In addition to verify late blight, the specific clonal lineage can be identified, and this is important, since isolates can vary in their aggressiveness to potato and tomato.