Education Center | Plant Disease Management Simulations
Management of Potato Late Blight: Simulation with Lateblight


Late Blight Home

Model Description

1. Disease resistance

2. Protectant Fungicides

3. Systemic Fungicides

4. Effects of weather

5. Disease thresholds

6. Sanitation

7. Certified Seed

8. Integrated Tactics

General Instructions

In order for the simulation to run on your computer, your browser must be Java-enabled. For a free download of the necessary software and the installation instructions, go to

Click the Start Simulation button to the left. The simulation opens in a new window, across the top of which is a menu bar. Check each menu and note the selections in each. The menu at the far right is a Help menu that gives you an explanation of each command and menu selection. There is also context-sensitive help for each menu item.

From the Potato menu, you can select cultivars with low, moderate, and high levels of resistance to Phytophthora, and from the Environment menu, you can select various weather options to see how late blight responds to temperature and moisture. In the Inoculum menu, you can enter values for the numbers of sporangia blowing into the field from cull piles or from adjacent unsprayed potato fields, and you can enter values for the number of infected tubers that are in your field from uncertified seed potatoes or from volunteers from the previous season. The Management menu allows you to check the weather forecast day by day and to spray either a protectant fungicide or a systemic fungicide as needed.

Select Begin New in the Simulation menu to start the simulation, and use the right arrow key or click on the right arrow of the scroll bar to advance the simulation through time.

The simulation behaves somewhat differently with different browsers and from one computer to the next. If the model does not load fully on the first try, close it and open it again. If the scrollbar just below the graph does not appear, increase the resolution of your monitor and/or enlarge the simulation window. If the help window will not appear, check to see that the browser's popup blocker is turned off for this application.

Note that the default units of measure are metric. If you prefer to work in acres, gallons, pounds, hundredweight, etc. select American in the Measure menu. (Select the desired units at the start of the simulation before clicking on Begin New. The metric/American conversion is not designed to work while the simulation is running.)

For a detailed description of how the simulation works and how the model is constructed, see the Lateblight Model Description.

....proceed to EXERCISE 1

Instructors: See Instructor Answer Sheet.


Late blight is a disease of potatoes and tomatoes caused by the fungus-like organism, Phytophthora infestans. Under favorable environmental conditions, and in the absence of any control measures, this disease can destroy virtually 100% of the above-ground parts of susceptible cultivars.

Inoculum produced on infected foliage can be disseminated to the tubers, where infections can also occur. Losses include reduction of photosynthetic area resulting in reduced bulking of tubers (yield loss), blighted tubers (quality loss), and decay of tubers in storage (postharvest loss).

Management of potato late blight begins with reducing the initial inoculum by:

  1. Keeping the numbers of infected seed tubers to very low levels by planting only certified seed
  2. Crop rotation to reduce the numbers of volunteer plants (which may be infected)
  3. Burying or composting cull piles, or at least keeping them far away from the potato fields.

Control of late blight by means of potato varieties with a high degree of resistance to P. infestans has not proven to be very practical because of the rapid selection of virulent races of the pathogen. Varieties with moderate levels of resistance, however, have been very useful when combined with other management tactics, particularly the use of fungicides. In most potato growing areas, regular fungicide application is an essential component of late blight management.

Because of environmental concerns, the problem of fungicide resistance, and the high cost of fungicides and their application, potato growers have been forced to reduce their overall use of fungicides and to improve the efficiency of the fungicides they apply. One approach has been to improve the timing and adjust the dose of fungicide applications according to environmental conditions. Various kinds of forecasting systems have been employed to help growers decide when and how much to spray. By using potato varieties that are partially resistant to P. infestans, it is possible to markedly reduce the doses and/or frequencies of fungicide sprays.

Arneson, P. A. 2005. Managing Potato Late Blight: Simulation with Lateblight. The Plant Health Instructor. DOI:10.1094/PHI/A-2005-0722-01.   This version of Lateblight was adapted from the simulation originally written by J. A. Bruhn et al. (1980) and modified in various ways over the years by different people working in the research program of W. E. Fry, Department of Plant Pathology, Cornell University. We have retained the original structure of the model wherever possible, but some small modifications have been necessary to enhance its pedagogical value. While the simulation is sufficiently realistic for teaching purposes, this version should not be trusted as a research tool or a management decision making aid. An earlier version of Lateblight was adapted as a Microsoft Windows application by Barr E. Ticknor and Phil A. Arneson. It was rewritten into Java and modified as a web application in 2002 by Joshua M. Goldfarb.

Last updated: July 9, 2004
Copyright 2002 Cornell University