Late Blight Control: What Are the Options?
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 extensive 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. We now have an unconfirmed report of late blight in western New York this week (June 14, 2000), and our weather conditions are dramatically different from 1999. The summary that follows was adapted in part from an article originally prepared by Rosemary Loria.
Destroy cull piles and volunteers. Growers are well aware of the importance of cull piles as potential inoculum sources. Remember that tubers in stone piles at the edge of fields can also be a problem. We are not aware of any late blight in the state as a carryover from 1999, but because volunteers are likely to be among the earliest sources of late blight inoculum in the field, it is essential that volunteers be destroyed as soon as they are seen. The goal is to prevent the development of green leaf tissue and subsequent infection and sporulation of the late blight fungus. Fields with volunteers should be plowed as early as possible after potatoes emerge. We have reports of potatoes volunteering in field corn planted to potatoes last year.
Seed tubers. We have not had any reports of late blight in seed lots destined for planting in New York. However, given that late blight occurred in some seed production regions of the US in 1999, we should expect that inoculum will be present in some seed lots. If you planted a suspect lot, scout those fields early and often.
Scout cull piles, volunteers, and emerging fields. 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.
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, 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 currently have two other products that should be considered. Both Quadris and Curzate have preventative, curative and locally systemic activity. The key is that they are most effective if used preventatively. In both cases, it is important to follow label directions to prevent fungicide resistance from developing. Quadris (azoxystrobin) is registered for use on both potato and tomato. It will provide control of late blight if used preventatively, and 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 Bravo or another late blight effective material. Curzate 60DF is registered for use only on potato at the rate of 3.3 oz/A and must be tank mixed with a protectant fungicide such as Bravo or Manzate. Again it should be used on a 5- to 7-day schedule. 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.
Unfortunately, two other materials that are important for late blight control, Acrobat MZ (dimethomorph + mancozeb) and Tattoo C (propamocarb + chlorothalonil) are not currently registered for use on potato or tomato in New York. Acrobat MZ has not been approved by the DEC in Albany, but it is registered nationally for use on potato only. Tattoo C is not registered nationally, so a Section 18 request has been applied for and recently approved in three western states. We are currently applying for an Emergency Crisis Exemption for Tattoo C here in New York on potatoes, given the disease-favorable conditions that currently exist. Several years of data from Bill Frys program demonstrate that Tattoo C can suppress leaf lesion expansion, which is consistent with its suppressive effect on an already established epidemic.
For organic growers, several fixed copper fungicides are available (Basicop, Champ, Kocide, etc.) and provide fair control of late blight and early blight, again if used preventatively. These and other copper products are registered for use on both potato and tomato.
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.
Initiating fungicide applications. Fungicide applications can be initiated based on potato plant growth (4-6 inches in height), or based on weather information. Many studies have shown that late blight forecasts are useful for initiating fungicide applications, if monitoring begins when the first green tissue appears. Using this system, normally protectant fungicide applications are initiated (on plants 4 inches high or larger) when 18 severity values have accumulated. It is likely that this severity unit threshold has already been exceeded in most regions of the state, and fungicide sprays need to be started immediately. IF LATE BLIGHT IS IN THE VICINITY, begin fungicide applications as soon as the plants emerge.
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.