Online Reporter
LATE blight is a destructive fungal disease that affects horticultural crops in the solanaceae or nightshade family.
This foliar disease has been threatening the agricultural sector for centuries particularly targeting crops such as tomatoes and potatoes. Currently, Late blight is still affecting global food security and has been a thorn in the flesh of farmers resulting in significant economic losses. This article delves into the world of Late blight, examining its causes, symptoms and factors that influence its transmission and spread specifically highlighting its economic importance to the agricultural sector and the nation at large.
Causes
Late blight is a fungal disease caused by a pathogen — Phytophthora infestans. The spread and transmission of Late blight is influenced by the following factors;
Environmental factors
Wind
It can facilitate infection by blowing spores onto wet leaves and stems. Strong winds can transport spores from one host to another.
Moisture
Excess moisture from rainfall, dew, and irrigation creates a favourable environment for spore germination. Planting in waterlogged soils can also promote the germination of spores.
Temperature and humidity
Late blight spores survive well in humid conditions that are above 80% and this can facilitate spore dispersal increasing the chances of infection. Temperatures above 10oC can also promote disease development.
Host Plant factors
Weak/Susceptible varieties
There are certain varieties that have poor genetic makeup up predisposing the plants to infection. These varieties lack resistance genes that help protect against the disease resulting in a higher risk of disease spread.
Plant stress
The defence mechanism of stressed plants is very weak, exposing the plant to infection. Plants under severe stress tend to release more ethylene, a hormone that promotes and stimulates spore germination. The inability of stressed plants to photosynthesise decrease the energy availability for defence against Late blight infection.
Age of the plant
Since young plants are more succulents due to their tender tissue and high moisture content, this create a large surface area for infection. During the vegetative stage of plants, there is rapid cell division which makes them susceptible to Late blight. At a younger stage of plants, the defence mechanism is poor, increasing their vulnerability to infection.
Farming practices
Over-irrigation
It creates a conducive environment for spore germination.
Ignorance
Misuse of fungicides can promote resistance development increasing the vulnerability of plants to infection. Failure to dispose of infected plants and debris can spread the disease.
Crop Rotation
Failure to adhere to principles and guidelines of crop rotation can lead to build-up of inoculum.
Socio-economic factors
Lack of awareness on the economic importance of Late blight can contribute to its spread. Shortage of resources to manage the disease can be a barrier in Late blight mitigating strategies. The global trade of infected plants and products can also facilitates the spread of infection.
Stages of Symptom development
Stage 1 — Infection and Incubation
Fungi spores germinate on the tissues of the host particularly on the lower leaves. The germ tube forms an infection thread which penetrates the cell wall of the plant. The pathogen begins to dominate within the plant tissue producing enzymes that break down cell walls and multiplies without producing visible signs and symptoms. Breaking down of cell wall continues, resulting in cellular damage.
Stage 2 – Symptom appearance and infection progression
The first apparent signs and symptoms appear as water — soaked greyish — green lesions on lower leaves. These lesions expand rapidly becoming large and irregularly shaped. A white cottony growth shows up on the underside of the infected tissue. The growth of lesions continues with irregular patches leading to defoliation reducing the surface area for photosynthesis. The infection progresses resulting in low yields, poor quality fruits and tubers.
Stage 3 — Plant Death (severe infection)
Infected plants may seriously defoliate leading to plant death, rotten fruits and tubers that are inedible. Under serious infection, plants collapse causing a significant loss of yield.
It is important for horticultural farmers to have a know-how of the development stages of late blight as this can give them time to mobilize resources to reduce late blight spread. These symptoms can be summarised as follows;
Watery-soaked, greyish-green lesions on lower leaves.
A white cottony growth on the underside of the leaves.
Premature defoliation of diseased plants results in yield reduction.
Fruit and tuber infection reducing market value.
Complete death of the entire crop in severe infection resulting in significant economic loss.
Beyond yield reduction and crop losses, late blight has also economic and environmental impacts.
Economic Impact
Late blight can pose a threat to food security. Crop failures result in food unavailability, increased prices which affect consumer affordability. The cumulative impact of late blight on horticultural industry can slow down economic growth, adversely affecting gross domestic product (GDP). To prevent the spread of the disease, importing countries may impose restrictions, leading to trade disruption.
Environmental impact
Mitigating strategies of late blight mostly depend on the use of fungicides which can pose harm to soil and water. Continuous and excessive use of fungicides can accelerate resistance development. Farmers may be driven to convert marginal land to agricultural use leading to loss of habitat and fragmentation.
Repeated use of fungicides, can result in soil nutrient depletion, impacting long-term ecosystem health.
By understanding the impacts of late blight, players in the agricultural sector need to develop effective mitigating strategies that are sustainable and friendly to the environment. Early and accurate scouting of late blight symptoms is key in preventing its spread. It is advisable to consult local agricultural extension workers, agronomists and other experienced farmers once you suspect late blight infection in your crops.
(Catch our next post for expert advice on effective management strategies coming next week!)
