CUCUMBER, MELON, PUMPKIN, AND ZUCCHINI DISEASE MODELS
POWDERY MILDEW OF MELON
TABLE OF CONTENT
Powdery mildew is a common disease of cucurbits under field and greenhouse conditions in most areas of the world. Although all cucurbits are susceptible, symptoms are less common on cucumber and melon because many commercial cultivars have resistance. This disease can be a major production problem. Quantity of yield is reduced due to a decrease in the size or number of fruit or a decrease in the length of the harvest period. Premature senescence of infected leaves can result in reduced market quality because fruit become sunburnt or ripen prematurely or incompletely. Such fruit have poor storability (winter squash), low soluble solids with consequent poor flavor (melon), poor rind color (pumpkin), and shriveled, discolored handles (pumpkin). Stress from disease can lead to imperfections on fruit rind such as speckling, raised indentations, and oedema. In addition, powdery mildew infection predisposes plants to other diseases, in particular, gummy stem blight.
Podosphaera xanthii (previously known as Sphaerotheca fuliginea and S. fusca) and Erysiphe cichoracearum are the two most commonly recorded fungi causing cucurbit powdery mildew. E. cichoracearum was considered to be the primary causal organism throughout most of the world before 1958. Today, P. xanthii is found more commonly worldwide. A shift in the predominance of these two fungi may have occurred or the causal organism may have been misidentified. P. xanthii is a more aggressive pathogen than E. cichoracearum. E. cichoracearum may have a lower temperature optimum since this species is found mainly during cooler spring and early summer periods and P. xanthii appears to progress most rapidly during the warmer months. The conidia (spores produced asexually) of E. cichoracearum and P. xanthii are difficult to distinguish and cleistothecia, which are sexual fruiting bodies (structures containing spores produced through sexual reproduction), have been observed less commonly. Consequently, these fungi have been confused. The name of the fungus frequently has been reported without valid confirmation. Criteria for differentiating these fungi using the conidial stage were not identified until the 1960s. The main criterion used is presence of fibrosin bodies in conidia of P. xanthii. Based on these criteria, P. xanthii was found to be the predominant fungus, rather than E. cichoracearum as previously claimed, in several countries. During recent surveys E. cichoracearum was found rarely and only at the start of disease development in New York and other eastern states.
White, powdery fungal growth develops on both leaf surfaces, petioles, and stems. It usually develops first on crown leaves, on shaded lower leaves and on leaf undersurfaces. Yellow spots may form on upper leaf surfaces opposite powdery mildew colonies. Older plants are affected first. Infected leaves usually wither and die. Plants may senesce prematurely. Fruit infection occurs rarely on watermelon and cucumber. Cleistothecia are dark brown, small (diameter of about 0.003 inches) structures that are barely discernable without a hand lens. They develop late in the growing season. The sexual spores within these structures are protected from adverse conditions.
Picture source: VegetableMDonLine
The primary initial inoculum is believed to be airborne conidia dispersed over long distances from places, where cucurbit crops are grown earlier in the year. Conidia remain viable for 7-8 days based on results from laboratory studies. The causal fungi are obligate parasites and therefore cannot survive in the absence of living host plants, except as cleistothecia. Possible local sources of initial inoculum include conidia from greenhouse-grown cucurbits, cleistothecia, and alternate hosts. Cleistothecia have been reported rarely in the United States; however, even when present they can be overlooked. Both mating types required for sexual reproduction have been found throughout the United States, including New York. Although P. xanthii and E. cichoracearum are described as having broad host ranges, strains of these fungi have been shown to be host-specific. The role of non-cucurbit hosts as sources of inoculum has not been investigated. Verbena, a common ornamental plant, could be an important source of inoculum, especially for cucurbits grown as a crop or transplants in the same greenhouse as verbena.
Powdery mildew develops quickly under favorable conditions because the length of time between infection and symptom appearance is usually only 3 to 7 days and a large number of conidia can be produced in a short time. Favorable conditions include dense plant growth and low light intensity. High relative humidity is favorable for infection and conidial survival; however, infection can take place as low as 50% RH. Dryness is favorable for colonization, sporulation, and dispersal. Rain and free moisture on the plant surface are unfavorable. However, disease development occurs in the presence or absence of dew. Mean temperature of 68-80°F (20°C-27°C) is favorable; infection can occur at 50-90°F !0°C-32°C) . Powdery mildew development is arrested when daytime temperatures are at least 100°F (28°C). Plants in the field often do not become affected until after fruit initiation. Susceptibility of leaves is greatest 16 to 23 days after unfolding.
Genetic resistance is used extensively in cucumber and melon, and has been incorporated into most other cucurbit crops. Most resistant squash and pumpkin varieties in the United States contain one or two copies of the same major resistance gene from a wild cucurbit. Genetic of resistance is different in cucumber and melon. Recently a decline in the degree of suppression achievable with resistant varieties has been detected indicating adaptation in Podosphaera xanthii. Successive cucurbit plantings should be physically separated or at least planted up-wind of older plantings because older plants can serve as a source of conidia. Fungicides containing antagonistic fungi for biological control have been developed.
Fungicides should be applied following detection through the DDS. Inspect plants weekly beginning in July and after fruit initiation (when plants become more susceptible). Examine upper and under surfaces of five older leaves at each of 10 sites or until symptoms are found. Initiate a spray program when symptoms are found. A spring planting of summer squash will become infected first; therefore, when available, it can be used as an indicator of when to begin scouting vine crops and later plantings of summer squash.
For a preventive schedule, applications should begin when plants start to run and/or to produce fruit and conditions for infections are favorable. To obtain adequate control, fungicide is needed on the undersurface of leaves and on leaves low in the plant canopy because the fungus develops best on these surfaces. This can be best accomplished by using mobile materials (i.e. quinoxyfen, boscalid, triflumizole). Another approach is to improve efficacy of contact materials (i.e. chlorothalonil, copper) by maximizing spray coverage on undersurfaces of leaves. Air-assist sprayers are one of the most effective means for increasing coverage and deposits on all leaf surfaces. Coverage produced by traditional hydraulic boom sprayers can be increased by either decreasing nozzle spacing (10 inches is better than 20 inches), increasing volume (75 gpa has worked well), increasing pressure (at least 80 psi), or by changing to smaller nozzle tips that direct sprays at an angle to the canopy. Use water-sensitive paper to check spray coverage. Refer to the current Cornell Pest Management Guidelines for Commercial Vegetable Production for an updated list of available fungicides and follow label directions.
Development of fungicide resistance and consequent control failure is always a concern with mobile fungicides due to their single-site mode of action. Strains of the powdery mildew fungus resistant (insensitive) to such fungicides have been found throughout the United States. Reduced sensitivity to fungicides from several chemical groups has been detected in other areas of the world as well. Therefore, tactics should always be used to minimize the potential of resistant pathogen strains being selected: apply mobile fungicides with contact fungicides, apply them only when needed most to protect yield (which usually is at the start of disease development), use highest labeled rates, and alternate between mobile fungicides with different modes of action as indicated by their FRAC code when possible (triflumizole and myclobutanil have the same mode of action; they are in FRAC group 3). In addition, maximize spray coverage and also use nonchemical control practices. At the start of powdery mildew epidemics, the frequency of strains resistant to mobile fungicides usually has been sufficiently low that at least one application of these fungicides has suppressed powdery mildew. This situation could change in the future. The frequency of resistant strains can increase rapidly following treatment.
Several biopesticides approved for organic production are registered for this disease in the United States. These products contain natural ingredients such as botanical oils, bicarbonates, hydrogen dioxide, and lipopeptides. They are contact materials, thus good coverage is critical for effective control. Products evaluated in university trials have exhibited a range in efficacy with some being as effective as conventional contact fungicides.
In FieldClimate.com the risk of Powdery mildew is detected by the sensors: leaf wetness and temperature. Conditions for optimal development of the fungal pathogen are:
- Rain and free moisture on the plant surface are unfavorable.
- Mean temperature of 68-80°F (20°C-27°C) is favorable; infection can occur at 50-90°F !0°C-32°C). Powdery mildew development is arrested when daytime temperatures are at least 100°F (28°C).
- Susceptibility of leaves is greatest 16 to 23 days after unfolding.
Conditions on the 20th of July as well as at the beginning of August have been favorable for the fungal pathogen. During these periods the temperature was between 20°C and 27°C and it was quiet dry (no leaf wetness periodes).
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TOMCAST (TOMato disease foreCASTing) is a computer model based on field data that attempts to predict fungal disease development, namely Early Blight, Septoria Leaf Spot and Anthracnose on tomatoes. Field placed data loggers are recording hourly leaf wetness and temperature data. This data where analysed over a 24 hour period and may result in the formation of a Disease Severity Value (DSV); essentially an increment of disease development. As DSV accumulate, disease pressure continues to build on the crop. When the number of accumulated DSV exceed the spray interval, a fungicide application is recommended to relieve the disease pressure.
TOMCAST is derived from the original F.A.S.T. (Forecasting Alternaria solani on Tomatoes) model developed by Drs. Madden, Pennypacker, and MacNab? at Pennsylvania State University (PSU). The PSU F.A.S.T. model was further modified by Dr. Pitblado at the Ridgetown College in Ontario into what we now recognize as the TOMCAST model used by Ohio State University Extension. DSV are: A Disease Severity Value (DSV) is the unit of measure given to a specific increment of disease (early blight) development. In other words, a DSV is a numerical representation of how fast or slow disease (early blight) is accumulating in a tomato field. The DSV is determined by two factors; leaf wetness and temperature during the „leaf wet“ hours. As the number of leaf wet hours and temperature increases, DSV accumulate at a faster rate. See the Disease Severity Value Chart below.
Conversely, when there are fewer leaf wet hours and the temperature is lower, DSV accumulate slowly if at all. When the total number of accumulated DSV exceeds a preset limit, called the spray interval or threshold, a fungicide spray is recommended to protect the foliage and fruit from disease development.
The spray interval (which determines when you should spray) can range between 15-20 DSV. The exact DSV a grower should use is usually supplied by the processor and depends on the fruit quality. Following a 15 DSV spray interval is a conservative use of the TOMCAST system, meaning you will spray more often than a grower who uses a 19 DSV spray interval with the TOMCAST system. The trade off is in the number of sprays applied during the season and the potential for difference in fruit quality.
Studies have been initiated at Michigan Staate University to test the disease forecasting system, TomCast, for use in managing foliar blights on carrot. TomCast has been used commercially in tomato production, and has recently been adapted for use in disease management of asparagus. Processing carrots ‘Early Gold’ were planted with a precision vacuum seeder at the MSU Muck Soils Research Farm in three rows 18 inches apart on a raised bed that was 50 feet long. Carrot beds were spaced on 64 inch centers and inrow seed spacing was 1 inch. Each of the four replications of the experiment were located in separate blocks of carrots that consisted of 36 beds. Seventeen treatment beds 20 feet long were randomly placed in a checker board pattern in each replication. Treatments were applied with a CO2 backpack sprayer that was calibrated to deliver 50 gallons per acre of spray solution using 8002 flat fan nozzles. Treatments consisted of an untreated and different schedule applications of Bravo Ultrex 82.5WDG (22.4 oz/A) alternated with Quadris 2.08SC (6.2 fl oz/A). The chemical program was applied on a 10 day calendar program as well as when predicted by the TomCast disease forecaster. Three different prediction thresholds of 15, 20, and 25 DSVs were used to time fungicide applications. When the cumulative daily DSV values reached the determined threshold a spray would be applied. Each treatment regime was initiated at four different levels of disease pressure (0%, trace, 5%, and 10% foliar blight). The first treatments were applied on 2 July and the last application of any treatment was made on 21 September. Ten feet of each center row of the spray blocks were marked before the first application and were used for weekly disease ratings (see graphs, below). Yields were taken from the same ten feet section of row by hand harvesting the carrots and topping and weighing.
This indicates that the first treatment in carrot should be done as soon as we can find the first disease incidence in field. From now on it worked fine by the use of the TomCast model with a threshold of 20 DSV accumulated since the last spray.
Fieldclimate.com determines the severity of an Alternaria Infection in two different models:
Source: (Jim Jasinski, TOMCAST Coordinator FOR OHIO, INDIANA, & MICHIGAN)
In dependence of the climatic conditions of hours of leaf wetness and air temperature, values of severity of an Infection (from 0 – 4, see table above) are determined.
PHYTOPHTORA INFESTANS NOBLIGHT
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Potato late blight is one of the most destructive foliar diseases on potatoes and has been reported for more than 150 years. Few plant diseases result in the widespread misery and despair produced by potato late blight. Potato late blight is caused by Phytophthora infestans; a fungus-like organism that over seasons in infected tubers, cull piles, and in infected volunteer plants. Potato late blight is a community disease and continues to pose a threat. All potato growers should monitor their fields continually for this disease. The main sources of initial inoculum are cull piles or infected seeds. The most effective—as well as cost-effective— way to control this disease is through control of initial inoculum. For this reason, growers should give careful attention to all sources of inoculum including seed, cull piles, rock piles, and other sources of volunteer potatoes. The ability of the pathogen to travel long distances dictates that a protectant spray program is needed.
Late blight control in Maine depends on proper application—timing, rate, and coverage—of protectant materials. The use of predictive models can permit late blight control with fewer, timelier chemical applications, which will help control costs and reduce chemical inputs to the environment.
Assessing the potential for late blight: Fungicide applications to control late blight should be based on weather conditions, not on a calendar. In most years, a calendar-based program applying fungicides weekly may start fungicide applications earlier than needed. In many years, portions of the growing season may need fungicide applications more frequently than once per week, while other portions of the growing season may need fungicide applications less frequently than once per week. Application of late blight control materials should be based on a predictive model in order to be efficient and effective.
In Maine, the potential for late blight to appear is predicted with severity values. Severity values are based on weather conditions and accumulate when they are appropriate for the development of the pathogen. The environmental conditions conducive to late blight development are generally mild and wet. The computer model “NoBlight” was developed in Maine and is used to guide the initiation and subsequent applications of fungicides for control of potato late blight in Maine.
Blitecast (a form of NoBlight model), which uses Wallin’s model of severity value accumulation. Wallin severity values are derived from various combinations of the hours with a relative humidity of 90 percent or greater and the average temperature during those periods. The duration of continuous periods of relative humidity of 90 percent or greater is tracked and the average temperature during these periods is calculated. Severity values are assigned based on these measurements and calculations and are accumulated in the manner demonstrated in Table 1. The first occurrence of late blight is predicted seven to ten days after 18 severity values have accumulated. The NoBlight model initiates accumulation of severity values starting at 50 percent plant emergence.
NoBlight like Blitecast, weights relative humidity more heavily than rainfall in predicting the timing of the applications. Close study of Table 2 will reveal that the spray interval becomes shorter with the accumulation of 25 mm (1.18 inches) of rain over the previous seven days under the same number of accumulated severity values.
Source:Steven B. Johnson, Extension crops specialist, UNIVERSITY OF MAINE COOPERATIVE EXTENSION
NoBlight differs from Blitecast in the accumulation of severity values based on relative humidity. NoBlight does not stop accumulating conducive conditions where the relative humidity drops below 90 percent. Blitecast uses 76.5 percent relative humidity to discontinue accumulation of conducive infection conditions.
Usually, this adds a half hour or more onto the typical Wallin hours. Typically this is a dewy morning period in Maine summers. More importantly, this does not discontinue the accumulation of conducive conditions when the relative humidity drops to 88 percent for a period of time. In effect, the severity values accumulated by NoBlight are more conservative that the Wallin severity values. As can be seen from Table 1, three separate six-hour periods of relative humidity greater than 90 percent will not accumulate any severity values.
However, an 18-hour period of relative humidity greater than 90 percent will accumulate severity values, depending on the average temperature during that period (3 severity values at 18.3 °C (65°F), 2 at 13.3 °C (56°F), 1 at 10 °C (50°F), and 0 at 4.4 °C (40°F) or 29.4 °C (85°F)). Once 18 severity values have accumulated after emergence, a protective fungicide application is recommended. After that time, the recommended application interval is based on additional severity value accumulation during the previous seven days in the manner described in Table 2. Fungicide treatment for the prevention of late blight should begin immediately if the disease is developing from seed or has otherwise been sighted in the field or nearby fields.
As with any model, NoBlight is no better than the data it analyses. The value of a predictive model is to provide the user with a reliable estimate of when conditions are conducive for late blight development and when conditions are not conducive for late blight development. The model provides some guidance on when a grower can stretch spray intervals with minimal risk, as well as when the spray interval needs to be reduced because the crop is at risk.
In FieldClimate.com the Disease Severity Level (from 0-4 ) are determined in relation to the conditions of precipitation, rel. humidity and air temperature.
Following the above described calcualtions of severity levels (see table) the spray interval is adapted to that intervals and shortended for example on the 29th of July to former 12 days to 10 days and on the 30th of July again to an interval of 7 days. On 2nd of August the conditions for the fungal pathogen have been favourable again and a disease severity value of 1 has to be accumulated and so a spray interval of 5 days is recommended.