Angular leaf spot (ALS) is a disease that poses a threat to cucurbit crops, which include cucumbers, melons, squash, and pumpkins. Caused by the bacterium Pseudomonas syringae pv. lachrymans, the disease is identifiable by the appearance of distinct, water-soaked lesions on the leaves. These spots are initially small but expand until they are contained by the leaf veins, giving them a sharp, angular shape.
The Crucial Role of Moisture
Moisture is the primary environmental factor that triggers the infection cycle of Angular Leaf Spot. The bacteria require a period of prolonged leaf wetness to successfully penetrate the plant tissue. This moisture allows the bacteria to enter the leaf through natural openings, such as stomata, or through small wounds.
High relative humidity, typically exceeding 80% to 90%, is important because it prevents the leaf surface from drying out quickly. This prolonged wetness creates the necessary aquatic bridge for the bacteria to move and initiate the infection. When humidity is high, a clear to milky bacterial ooze forms on the lesions, which dries into a thin, whitish crust and serves as a secondary source of infection.
Splashing water, whether from heavy rainfall or overhead irrigation, plays a direct role in the physical transmission of the pathogen. Water droplets striking infected debris or leaves can aerosolize the bacteria, carrying them onto healthy foliage. The practice of using overhead sprinklers, which keeps leaves wet for extended periods, is particularly conducive to rapid disease spread.
Optimal Temperature Requirements
While moisture triggers infection, temperature dictates the rate of bacterial growth and symptom severity. The bacteria thrive within a moderately warm range, typically 75°F to 82°F (24°C to 28°C), for optimal multiplication. The most damaging outbreaks occur when temperatures are specifically around 77°F (25°C) to 80°F (27°C). Temperatures significantly outside this window, such as above 98.6°F (37°C), slow or halt the progression of the disease.
Disease severity is a function of the co-occurrence of moderate temperatures and high moisture. A combination of warm days and warm, moist nights is especially favorable for a disease epidemic. Although the bacteria can survive in cooler or warmer conditions, the most damaging outbreaks occur when temperatures are within this ideal range.
Environmental Factors Aiding Disease Spread
Beyond the basic requirements of moisture and temperature, several other environmental factors facilitate the movement and long-term survival of the ALS pathogen. Wind is an important dispersal agent, capable of carrying both infected water droplets and bacterial aerosols over short distances to new plants. On sandy soils, wind-blown sand can also cause microscopic abrasions on leaf surfaces, creating easy entry points for the bacteria.
The presence of infested crop debris is a significant environmental reservoir for the bacteria, which can survive in the soil or on dead leaves for over two years. Environmental conditions like humidity and soil moisture allow the bacteria to persist in a dormant state until a susceptible host is planted. When rain or irrigation splashes soil onto new seedlings, the bacteria are physically moved from this debris to the healthy plant.
Certain cultivation practices that interact with the environment also promote the disease, such as working in the field when the foliage is wet. The physical contact of farm equipment, tools, or workers with wet, infected leaves easily spreads the bacterial ooze from plant to plant.