Early blight is probably the most common and widespread of foliar diseases in field-grown tomatoes in New England. Contrary to what its name suggests, this disease normally appears late in the season, as plants develop a full load of ripening fruit. In our disease management trials at the University of Massachusetts, and in farm fields, the first symptoms usually develop in mid to late July. However, it is not until mid- to late August that we see rapid expansion of the disease throughout the foliage. Uncontrolled, early blight can completely defoliate tomato plants and cut short the harvest. In four years of trials, we have seen reductions of 9% to 52% in the total marketable fruit due to early blight, in unsprayed plots.
Alternaria solani, the organism that causes early blight, is a fungal pathogen that attacks several solanaceous crops including potato and eggplant. Early blight disease begins when Alternaria spores (conidia) land on the leaf surface, germinate, and penetrate the leaf tissue. Fungal mycelia grow and expand, producing characteristic brown, circular lesions with dark concentric rings like a target board. These lesions produce new spores, which are spread by wind or running water, or carried by workers or implements onto new foliage. Under favorable conditions, this complete cycle -- from a spore landing on a leaf to production of thousands of new spores -- takes about one week. This rapid reproductive cycle explains why the disease can expand so rapidly and defoliate a crop within a matter of weeks.
What constitutes "favorable conditions"? There are two key factors: temperature and leaf-wetness. The leaf surface must be wet in order for spores to germinate. Growth of lesions and production of new spores are also favored by leaf moisture. In both cases, the longer the period that leaves are wet, the better -- for the pathogen. Temperature determines how fast these events happen. The fastest development occurs in the range between about 65 0F and 85 0F. Below 65 0F, development is slower, and below 550F, very little activity occurs. Weather conditions that create long periods of dew at night, with night temperatures above 65 0F -- and especially, above 70 0F -- are the most favorable. In New England, these conditions occur most often during July and August. Long periods of rain also favor the disease, if temperatures are warm. Generally, the same conditions that are good for early blight also favor another key fungal disease, Septoria leaf spot.
Susceptibility is also a factor in how successfully and rapidly the pathogen grows and reproduces in the plant. As plants age and the fruit load grows, nutritional demands are higher, and tomatoes become more susceptible to early blight. Early blight invades the older, less vigorous leaves first, and moves up the plant to younger leaves. Maintaining good crop vigor throughout the season, with adequate water and nitrogen, helps plants resist early blight. Genetic resistance also can be important. Early varieties tend to be more susceptible.
The first line of defense against any disease is producing healthy plants and growing them in an environment that is as free of the pathogen as possible. The fungus can be seed-borne, so it is important to start with clean seed. Alternaria solani is so widespread that it is difficult to escape it completely, but several cultural practices can help. We know that the fungus overwinters in infected crop debris and can persist from one season to the next in the same field. However, it will not survive in the soil during a rotation of 3 years or more into a non-host crop.
Managing disease in tomato, or any crop, starts with your selection of seed and your overall farm plan for crop rotation, and doesnt end until you have turned under the last of your crop debris and planted a cover crop. No single practice will give you adequate control, but an integrated approach from start to finish will shift the balance in favor of healthy, productive tomatoes for the full season. Table 1 lists a range of cultural practices that can reduce the incidence of early blight --as well as other key diseases such as Septoria leaf spot and bacterial canker.
|Table 1. Cultural Practices to Reduce Early Blight||Reduces:|
|Initial inoculum||Disease development|
|Thoroughly turn under tomato crop debris, preferably in fall||x|
|Use land that has been out of tomato, potato, or eggplant for at least 2-3 years; isolate current tomato plantings from potato and eggplant fields||x|
|Evaluate soil organic matter and nutrient status with an annual soil test||x|
|Adjust soil amendments to soil nutrient status and expected nutrient uptake by the crop||x|
|Provide for adequate, season-long nitrogen supply||x|
|Use high quality, disease-free seed||x|
|Select varieties that are less susceptible to disease||x|
|Set out healthy seedlings - avoid overgrown or severely wilted transplants||x|
|Use high tunnels for early crop to limit leaf wetness (no dew or rain; drip irrigation)||x|
|Reduce early season stress from wind, drought or cold, using high tunnels, row covers or wind breaks for early crop||x|
|Work in fields when foliage is dry||x|
|Isolate early plantings from later crops to reduce spread of disease||x|
|Plant a second, later crop using a later seedling date and/or later-maturing variety||x|
|Control weeds to provide good air circulation||
A 7-10 day spray schedule with protectant fungicides is a traditional and effective system for early blight control in tomato. The goal of this fixed-schedule spray program is to maintain a residue of protectant fungicide on the foliage at all times, so that when fungal spores germinate or fungal mycelia absorb the fungicide, their growth is stopped. With a calendar-based schedule, fungicide is applied often enough so that the residue never degrades beyond the point where it is sufficient to achieve control. Under this system, fungicides are applied anywhere from 8 to 12 times per season.
However, since weather conditions often do not favor the germination of Alternaria spores or the development of lesions and new spores, it is not really necessary to maintain such a high level of chemical residue in order to obtain disease control. The key to altering a fixed schedule is monitoring the weather conditions that affect disease development -- temperature and leaf wetness -- and making sure that protectant fungicide is present on the leaves when disease risk is high. For the past four years, the University of Massachusetts Tomato IPM Program has been evaluating TOM-CAST, a disease forecasting model for tomatoes. This system has been implemented successfully by growers in major tomato-growing areas of Ontario, Ohio, Iowa, Michigan, New York, New Jersey, Pennsylvania and California. Our results show that it can also work well in New England.
In TOM-CAST, a Disease Severity Value (DSV) is calculated daily, based upon the hours of leaf wetness and the average temperature during the leaf wetness period. Daily values range from zero (conditions unfavorable) to four (conditions very favorable for early blight). Daily DSVs are summed to produce a cumulative DSV value. This cumulative value serves as a threshold to trigger the next fungicide application.
It is important to note that this model applies to management of early blight as well as Septoria leaf spot, but does not apply to bacterial canker. Canker has a very different biology and is not influenced by the same weather conditions. If canker is present, copper sprays are recommended, and regular spray intervals are needed. Also, this system assumes the use of a fungicide containing chlorothalonil. Because mancozeb residue tends to be less persistent on the foliage than chlorothalonil, a lower DSV threshold (such as 12) may be needed if mancozeb fungicides are used.
Because young plants are less susceptible to early blight, fungicide protection is less critical early in the season. Therefore, the trigger for the first fungicide is higher than for subsequent sprays. TOM-CAST recommends the first spray when cumulative DSVs reach 35, or during the first week in July, whichever comes first. Typically, the first fruit is full grown or ripening at this time. In two of the past three years, cumulative DSVs reached 35 during the first week of July the two triggers occurred in the same week. In one exceedingly dry year (1993), 35 DSVs did not accumulate until late July.
After each fungicide application, cumulative DSVs return to zero. Subsequent fungicide applications are made when cumulative DSVs reach 15. When we have long leaf wetness periods and warm nights, - which typically occurs for at least a week or two sometime during July or August, we log daily DSVs of 2 or 3 and can reach this threshold in as little as 7 days. When we have a long period of dry air or night temperatures are below 55 0F, DSVs are often 0 or 1, and the spray interval may be stretched to over 21 days. It is also common to have a stretch of varied conditions that give daily DSVs of 0, 1, 2, or 3, and to reach the threshold in 10 or 11 days.
We have operated two weather stations each year for the past 3 years. One has been located at the UMASS Research Farm along the Connecticut River, and the other in eastern Mass. at Northborough or Acton. Assuming the first spray would be put down on July 4 and the last during the second week of September, TOM-CAST has recommended from 4 to 8 fungicide applications per season. Compared to 10 weekly sprays during the same period, this is a savings of 20-60% in fungicide and application costs. As I review the accumulation of DSVs in these 6 examples, I have to say that the only clear pattern that emerges is that the need for tightest spray intervals (7-10 days) typically occurs sometime between July 4 and August 20. However, when in this period they occur varies tremendously from year to year.
Does TOM-CAST produce good disease control and good yields? In three out of four years, trials comparing TOM-CAST with weekly sprays resulted in no reduction in marketable yield even with 40-50% fewer fungicide applications (see Table 2). These trials were conducted with the short-season variety Sunrise, under variable conditions such as rotated or non-rotated fields and ground or stake-and-weave culture. In all trials, treatment plots were surrounded by untreated border rows in which early blight reached epidemic proportions. In 1993, when total yield was 18% lower in TOM-CAST, disease pressure was especially high (unsprayed plots had 52% reduced yield) and plants were drought-stressed. This suggests that a reduced-fungicide system such as TOM-CAST will work best as part of an integrated management program that uses the best cultural practices as well.
Our efforts to evaluate TOM-CAST on growers fields have been complicated by the presence of bacterial canker. In 1995 at Stonefield Farm in Acton, fungicide applications were made at 15-20 DSV intervals in fields that were, for the most part, canker-free. Weather conditions were monitored on the farm. With a total of five fungicide applications, excellent disease control was achieved and foliage remained healthy into late September when vines were pulled up.
Ideally, every grower would be able to monitor temperature and leaf wetness on his or her own farm. However, most growers are unlikely to invest $1500 or more in the required equipment. Still, we believe that TOM-CAST can be a useful decision-making tool if weather conditions are monitored on a regional basis. Currently we issue reports from two stations, and have observed that the patterns are not radically different even between stations in two different parts of the state. We are exploring ways to expand our network to additional sites using electronic weather services.
Table 2. Results of University of Massachusetts TOM-CAST Trials, 1992 - 1995
Total Yield of Marketable Fruit. and Total Number of Fungicide Applications per Season
|Fungicide Treatment||No of applic||
|No. of applic||Yield (ton/A)||No. of applic||Yield (ton/A)||No. of applic||
|2. TOM-CAST 15z||6||33.9A||4||20.3 B||6||26.9 A||5||42.2 A|
|3. TOM-CAST 10y||-||-||-||-||8||28.8 A||-||-|
|4. Unsprayed||0||27.8B||0||11.9 C||0||19.0 B||0||38.1 B|
x Except in 1992, weekly sprays
were begun in the first week of July. 1992 first spray was June
z Threshold = 15 DSV; initial spray at 35 DSV or first week of July.
y Threshold = 10 DSV
1 Means in each yield column which are followed by the same letter are not significantly different from each other (p < 0.05, Duncan's).
Ruth V. Hazzard, Vegetable
University of Massachusetts, Department of Entomology, Amherst, MA 01003
Other articles on Early Blight
More vegetable IPM articles
Originally published: Proceedings. 1995 New England Vegetable and Berry Conference and Trade Show. December 12-14, 1995. Sturbridge Host Hotel. Sturbridge Massachusetts.
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law.Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations.The user of this information assumes all risks for personal injury or property damage.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kirklyn M. Kerr, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.