A healthy durian tree can withstand violent winds and heavy rains—yet a microscopic pathogen can bring it down in weeks. Diseases like Phytophthora target vulnerable plants, exploiting their weaknesses and turning minor stress into fatal infections. So how can you stop this from happening?

Signs and symptoms

One of the most prominent pathogens in the durian crop is the oomycete Phytophthora palmivora, which can cause up to 30% yield loss in an infected plantation (Drenth and Guest, 2004). It is a fungal-like organism causing diseases on multiple areas of the trees, such as patch canker, fruit rot, root rot and leaf blight. Here are some symptoms listed below:

• Patch canker: Black, wet-looking patches of dead bark, showing reddish brown tissues underneath.
• Fruit rot: Watery patches on fruit surface that develop and then dry out, turning dark with whitish or grey mycelia on the lesion. 
• Root rot: Dark brown, water-soaked roots. Plants show stunted growth and new leaves show yellowing and wilting. 
• Leaf blight: Brown, water-soaked lesions on leaves. Advanced stages of infection can lead to dieback.

Figure 1: Black lesion of patch canker (left). Leaf blight leading to dieback (right). 

How does the disease spread?

Figure 2: Life cycle of oomycete pathogens. (Singh et. al., 2025)

How should we approach this disease?

While chemical fungicides are able to control P. palmivora within infected hosts, the disease's damage makes the host weak, requiring a long recovery period before returning to pre-infection production levels. Furthermore, the long-term, frequent use of strong fungicides may lead to the pathogen building up a resistance to the fungicide, creating a possibility where current fungicides may not be enough to counter the disease’s effects. Rather than using strong fungicides for every incidence of infection, they should function as a last line of defence. Instead, methods which prioritize disease prevention and strengthening plant defence systems should be practiced. 

By preventing disease incidences, the occurrence and severity of the infection can be minimized, while strengthening the plant defence system leads to plants responding faster to diseases. 

Preventing disease occurrence: 

1. Choose locations which are not flood-prone or easily waterlogged. 
   P. palmivora spreads through water, so minimizing standing water in your farm can prevent infections. The location should also have well-draining soils and a source of clean water. 
   
   
2. Construct raised mounds for trees and dig trenches for drainage.
   Ensure that a good irrigation system is in place and avoid overwatering. This helps to direct unused water away, preventing waterlogging and lowering chances of infection. 

3. Ensure that abiotic factors are within an acceptable range.
   Abiotic stress weakens trees and makes them more susceptible to diseases. Hence, make sure that factors such as soil pH is within the 5.5 – 6.5 range, and that trees receive sufficient nutrients. This helps trees to combat abiotic factors beyond our control, such as extreme weather changes. 

4. Minimize the damage from other pests and diseases.
   Insects and animal pests damage trees and fruits, as well as bring in other diseases; this increases the chances of infection and disease spread. Proper management of pest populations can prevent widespread infections. 

5. Biosecurity measures while handling planting material and equipment.
   Clean tools between each use. Do not share tools between infected and healthy trees, and minimize unnecessary wounding of trees. Planting material such as soil and seeds should be stored in clean, dry areas to prevent pathogen growth. 

Increasing plant defence response:

Plants generally become more susceptible to diseases when they are weak and do not have enough energy for their defence system to fight against pests and diseases. The fruiting stage, recovery stage, and environmental stresses are some examples of what take up a lot of energy in plants . For instance, in the fruiting stage, most of the plants’ energy is directed toward forming fruits, making it easy for diseases to attack. Similarly, environmental stress causes plants to focus more energy on nutrient uptake, making them more susceptible to disease. As a result, plants are easily infected and yields drop drastically.

The resistance of a plant towards pathogens relies on it being able to detect the pathogen’s presence and respond to it quickly. Pathogens will possess or release specific chemicals which can be detected by specific plant receptors. Upon detection, it triggers a defence response in the plant in order to control the infection. Plants which do not respond quickly enough will succumb to the infection. 

Plant defence responses can be enhanced through the application of potassium phosphite. Potassium phosphite stimulates plant natural defences by pre-activating defence components such as signal molecules, so that the plant can react faster and stronger against pathogens when an invasion occurs. 

By doing so, infection rates can be decreased, and the severity of disease damage will be reduced. Plants will be healthier and crop productivity can be maintained, thus helping you avoid high yield losses. 

What is AB Phos?

AB Phos is a liquid product containing potassium phosphite, aiming to strengthen the defence system of plants against oomycete diseases such as Phytophthora palmivora. Formulated with 41.7% phosphorus pentoxide and 27.8% potassium oxide, it is not only suitable for durians, but also for other crops facing oomycete diseases, such as nightshades, cucurbits and other fruit trees (e.g. root rot, downy mildew).

Benefits of using AB Phos 

1. Minimized yield losses: Quicker and stronger plant response against oomycete diseases helps to prevent severe disease damage, protecting crop yields. 
2. Increased nutrient absorption: Heightened response against oomycete diseases leads to improved root growth and tolerance to abiotic stress, allowing plants to absorb nutrients better.
3. Decreased reliance on chemical treatment methods: Enabling plants to defend themselves against oomycete diseases reduces the need for constant strong fungicide treatments, reducing the risk of fungicide resistance.

Figure 3: Before and after application of AB Phos.

Apply AB Phos once every two months to safeguard against oomycete diseases and minimize yield losses.

Table 1: Application dosage of AB Phos.* 

*This is just a general guideline. Consult your technical executive for more in-depth usage directions. 

Want to know more? Find your area technical executive here and contact them for more information or a site visitation today!

References

Drenth, A. and Guest, D.I. (2004) Diversity and management of phytophthora in Southeast Asia. Canberra: Australian Centre for International Agricultural Research.

Singh, A. et al. (2025) ‘Management of phytophthora and phytopythium oomycete diseases in Durian (durio zibethinus)’, Crop Protection, 190, p. 107086. doi:10.1016/j.cropro.2024.107086.