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Funded Project

Funding Program: Regional IPM Competitive Grants - Northeastern

Project Title: IPM for Aerial Dispersal Risk of Potato Late Blight IPM, Phytophthora infestans

Project Directors (PDs): Don Aylor [1] William Fry [2]

Lead State: CT Lead Organization: Connecticut Agricultural Experiment Station

Cooperating State(s): New York

Research Funding: $149,628

Start Date: Sep-01-1999 End Date: Aug-31-2002

Pests Involved: late blight, phytophthora

Site/Commodity: potatoes

Area of Emphasis: modeling, forecasting

Summary: Strategies for controlling Potato Late Blight with fewer applications of chemical pesticides require improved methods for predicting risk of infection. The major goal of this project is to improve understanding of the aerial dispersal of Phytophthora infestans sporangia and use this knowledge to predict infection probabilities and improve management of Potato Late Blight through integrated use of sanitation, scouting, weather forecasting, resistant varieties, and fungicides. The aerial transport of P. infestans sporangia to IPM managed fields from off-farm sources presents a potential risk of crop loss due to Late Blight infection that may preclude full implementation of IPM practices. We will develop a mathematical model and collect data that will be used to: 1) quantify the escape of sporangia from infected potato or tomato fields in terms of sporangia production, both in time and space, 2) develop an improved biophysical spore transport model incorporating Lagrangian simulation and K-theory (previous modeling efforts have used the Gaussian plume model with an assumed source strength) which incorporates temporal patterns of sporangia production, release and escape from the canopy, survival of sporangia during transport in the atmosphere, and deposition and infection probabilities to predict chance of infection as a function of distance from a source, 3) to use the improved model to evaluate the importance of outside inoculum sources compared to in-field (e.g., a low level of contaminated seed, or a volunteer plant) or near-field (cull piles) sources. These findings can then be integrated into a set of decision-rules for determining risk of blight infection from outside inoculum sources. Importantly, the model will allow an area-wide (regional) evaluation of disease spread potential and can be linked to other information sources either to give warnings or to suggest options for regional cropping plans. The results to be obtained here are complementary to, and help consolidate results from, other projects in the Northeast currently addressing integrated management of Potato Late Blight. The framework developed here should also be useful in evaluating risks of disease spread in other crop systems. Objectives: Our overall objective is to provide quantitative understanding for the production, escape from the crop canopy, and dispersal of sporangia of Phytophthora infestans (Mont.) de Bary for subsequent use in constructing more effective disease management strategies. This quantitative understanding will be organized by a system of models. There are three subobjectives: 1) quantify in time and space the escape of sporangia from infected potato or tomato fields as a function of sporangia production and environmental conditions, 2) develop improved and more realistic spore dispersal models which incorporate the effects of canopy microclimate, plant canopy structure, turbulence, and atmospheric stability to predict sporangia concentrations as a function of distance from a source, 3) use the models to evaluate the relative importance of outside inoculum sources compared to in-field (a low level of contaminated seed, or a volunteer plant) or near-field (cull piles) sources. Our long range goal is to contribute to the development of a forecast system that incorporates the probability of incoming aerial sporangia. This system will include predictions of the likelihood that infections in one field might produce sporangia that will be transported to another field within the region. If the danger of infections in an field is slight because there is little indigenous inoculum (because of sanitation, careful selection of seed, or other measures), then it may be possible to reduce pesticide use within the field by eliminating some sprays as long as there is little or no danger from exogenous inoculum. This system will utilize source strength (numbers of lesions and numbers of sporangia), probability of dispersal from the source, pathway of transport, survival during transport, and success rate of infections. Problem, Background and Justification PROBLEM Potato late blight, caused by Phytophthora infestans (Mont.) de Bary is one of the most destructive diseases of potato on a worldwide basis. The recent resurgence of the disease due to the arrival in the Northeast of more aggressive strains (including the second mating type) of the pathogen (31) underscores the need for renewed and improved management guidelines. Fungicides are an important component of the overall management strategy of late blight in the Northeast and in most temperate rain-fed production systems. Although resistant cultivars have been developed, market forces have limited their adoption and older, more susceptible, cultivars dominate production. The resulting need for fungicides in Late Blight control has stimulated a search for the most efficient methods to utilize fungicides. The importance of weather to disease development has long been recognized, and decision rules based on temperature and moisture conditions in the field have been developed and integrated into disease warning systems (22,23,38,41,57). In addition to variable weather, variation in the occurrence of the pathogen has contributed significantly to the sporadic nature of late blight. In many locations, in the United States during the 1980's, the pathogen was not detectable. Clearly, forecasts could be improved if they included knowledge of presence or absence of the pathogen in an area. As long as P. infestans remained exclusively asexual in a production region, there were two mechanisms by which the fungus could occur in a field: via infected tubers (either surviving from the previous year or unintentionally planted), and via aerial transport (presumably as sporangia) from outside sources. Long distance transport of infected tubers by people is well documented, and the several forecasting systems predict with varying accuracy the initial occurrence of late blight when infection comes from a soil-borne tuber. Currently lacking is a method to predict quantitatively whether sporangia from a source outside a field will be transported to that field, and if transported, whether those sporangia will initiate disease. BACKGROUND There have been only a few studies to suggest the distance that the sporangia could be dispersed (34). Van der Zaag (55) inferred indirectly that sporangia of P. infestans could be dispersed by wind at least 11 km without losing their viability. Recently, a blanket late blight infection on tomatoes was observed in central New York State in the summer of 1993, which covered about 40-60 km range from a disease focus (29). These studies are circumstantial and, as far as we are aware, there have been no papers that have actually quantified more long-distance dispersal. Several observations on the biology of the pathogen seem to suggest the presence of a limited horizon for late blight dispersal. There are two potential causes for the horizon. Firstly, the longevity of airborne sporangia could be limited due to exposure to inhospitable temperature and humidity and to UVB radiation. Minogue and Fry (45) showed that the average half-life of the sporangia exposed to air (but not to sunlight) was 5.5 hours at 15-20 C and 40-88 % of relative humidity, but this time was shorter when the temperature was relatively high. The effect of ultraviolet radiation on survival of sporangia has a dramatic effect on their effective dispersal distance (4). Recently, Mizubuti et al. (unpublished) showed that survival of P. infestans sporangia was decreased rapidly by exposure to sunlight. Substantial decreases in germination occurred in 15 min and germination was reduced by about 95% in one hour. Secondly, under some circumstances rainfall just after sporangia escape from the crop canopy could wash out virtually all the airborne sporangia, and could consequently prohibit their farther dispersal. Washout is a two-edged sword, however. Although fewer sporangia travel very far in rain, those that are deposited in rain presumably have an increased chance of causing disease. Risk of late blight infection depends on several factors including: the number of sporangia available for dispersal at any given time; the fraction of those available sporangia that can become airborne; dilution by the wind and removal of spores from the air by rain and by dry deposition; survival of sporangia during flight, the efficiency of deposition of sporangia on susceptible tissue; and the amount of susceptible host tissue per unit ground area (4,9,12). Needed is an aerobiological model of spore transport to integrate these several factors into an estimate of risk of infection from aerially dispersed spores (9). Sporangia production. As far as we are aware there have been no published studies quantitatively relating the "standing crop" of sporangia in a crop canopy to the escape of sporangia into the air, where they might become regional voyagers. Several studies have been determined sporangia production potential (21,47,48,54). We will go beyond these studies to quantitatively relate sporangia production to airborne sporangia concentrations in a field. The model to be developed here will complement our experiments by strengthening their biophysical interpretation and aims to replace the need for complex equipment and measurements with simpler observations. P. infestans can be a prolific sporangia producer; on tomato Legard et al found as many as 300,000 sporangia per lesion (42). Diurnal Patterns of Sporangia Release. Typically, sporangia of oomycetes are produced during a period of high humidity at night, and then are released mid morning (Fig. 1) -- presumably liberated by changing relative humidity (34,35). Fig. 1. Daily release patterns of Phytophthora infestans sporangia [after Hirst 1953 (35)] and Peronospora tabacina sporangia [after Aylor 1986 (4)]. The patterns for both Peronospora tabacina and Phytophthora infestans (Fig. 1) appear similar in that peak release occurs during mid-day. However, environmental conditions likely influence peak release, as exemplified by the two curves for P. tabacina in Fig. 1. These two curves mainly represent temporal differences in the drying potential (vapor pressure deficit) of the air on cloudy and sunny days. Because sunlight plays a major role in spore survival, differences in time of release can have a significant effect on dispersal distance (4). It is likely that such differences also exist for P. infestans, but were masked by taking geometric means of many days for the data portrayed in Fig. 1 (35). As an offshoot of our sampling protocol (described below) we will develop new information on this aspect of the aerobiology of P. infestans. Dispersal and Survival. Although most sporangia are deposited within a few meters of the source (56) some can be dispersed by the wind much farther, and perhaps for distances of several km (36). There are very few quantitative studies of the dispersal of P. infestans sporangia and (as far as we are aware) practically none for dispersal beyond 100 m. Thus, there is a great need for a quantitative model of sporangia dispersal and we intend to fill this gap. Quantitative data on survival of spores exposed to solar radiation outdoors are relatively rare. Only recently has quantitative information on survival of P. infestans sporangia exposed to sunlight been developed (Mizubuti et al. unpublished). P. infestans sporangia are highly sensitive to sunlight in comparison to other spores (Table 1, other data from Refs. 14, 20, and 49), suggesting a much shorter dispersal distance than for many other pathogens. Table 1. Time scale, tS, for survival of various spores exposed to sunlight. Germination, G, is the measure of spore survival, and critical exposure is defined by G(t)=G0 exp(-It/I*t*). (4) is accumulated in tS days. Pathogen...........Stage.........I*t*tSMJm-2..tS day P. infestans.......sporangia.....0.9.........~ 0.05 P. tabacina........sporangia.....2.4.........~ 0.1 V. inaequalis......conidia......21...........~ 1 U. appendiculatis..urediospores.25...........~ 1 A. solani..........conidia......35...........~ 2 This small value of tS is encouraging for IPM of Late Blight. However, there is a much longer time scale for survival of spores under cloudy skies (20,49), and this needs to be kept in mind. Therefore, it is critical to determine both the temporal pattern and amount of spore release on cloudy and sunny days. This will be one focus of this proposal. JUSTIFICATION The biological information and model of sporangia dispersal and infection probability that we will develop will help determine when regional levels of inoculum are sufficiently low to postpone or forego chemical control. Application of the model will enable a critical evaluation of the benefits - and potential pitfalls - of reducing fungicide sprays in Late Blight management. This research will contribute to the fields of biometeorology, epidemiology, and aerobiology by developing a methodology for directly linking spore production in a crop to the potential for aerial spore dispersal and risk of regional disease spread. It will further the goals of IPM in the Northeast by emphasizing the importance of scouting, and helping to evaluate the need for fungicide applications when inoculum levels are kept low by sanitation practices, use of certified seed, resistant cultivars, or other methods. This proposed work is complementary to other projects in the Northeast, and the model framework will be developed to readily accommodate new information as it is developed on the biology of the fungus and the degree of susceptibility to blight of various potato cultivars. Stakeholder needs: Late blight has emerged recently in the United States as a very important problem (30). It has been identified by growers and state departments of agriculture in the Northeast and also nationally, as a high priority problem needing multifaceted investigation (29). Additionally potato late blight has been recognized as a serious problem by the United States Department of Agriculture and they are currently searching for a scientist to be located in Maine to work on late blight. The National Potato Council has recognized late blight as a high priority problem in the Northeast and also nationally, and they have caused late blight to be added to the priorities needing attention in their USDA ARS potato research initiative. Finally, the agro-industry recognizes the significance of late blight and their emphasis is on the development of resistant varieties and new fungicides. Three fungicides were granted approval for emergency use (Section 18 registration) from 1995-1998. Two of these fungicides have recently gained Section 3 approval by the US EPA. Outcomes and Impacts Summary from 2001 IPM Center report To improve the management of potato late blight and reduce chemical pesticide applications, growers will require improved methods for predicting the risk of infection. Donald Aylor is working to strengthen our understanding of the aerial dispersal of P. infestans spores and is using this knowledge to predict infection probabilities. With better knowledge of late blight risk, growers can better control the disease through integrated use of sanitation, scouting, weather forecasting, late-blight-resistant potato varieties, and fungicides. This strategy could have an effect on more than 50,000 acres of potatoes, with the greatest impact in regions where there are several growers within a few miles of each other. If fully implemented with an intensive monitoring system, this new approach could allow growers to reduce sprays by 10 to 20 percent, which translates to a potential regional savings of more than $5 million.

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