White rust, also called white blisters, is a fungal disease caused by the fungus Albugo candida, which affects cruciferous plants that include vegetables such as kale, Brussels sprouts, cauliflower, broccoli and cabbage. Brassica weeds are also attacked.
Infection causes distinctive chalky white spore masses/pustules that first show up on the underside of leaves.
The disease causes significant economic losses due to damage to broccoli heads and leafy brassicas thus reducing market value, requiring more frequent fungicide applications, reducing production windows and slowing crop growth. In severe cases it can lead to total crop loss.
The pathogen commonly occurs when nights are cool and damp and days are warm. The perfect time for growing cruciferous vegetables also provides the perfect growing conditions for the fungus.
The primary infection occurs due to oospores perennating in the soil or due to mycelium perennating on perennial hosts. These serve as primary inoculum when the environmental conditions are favourable.
Oospores germinate in presence of water to form a vesicle in which a large number of zoospores are formed. These zoospores swim in a film of water and land on the suitable host, germinate by germ tubes, enter the host and establish infection.
After absorbing nutrients, the mycelium accumulates below the lower epidermis. Conidiophores, which are clavate and formed at the tip of hyphae begin to produce conidiosporangia in basipetal succession. The pressure of these breaks open the lower epidermis and white rust symptoms become apparent on the leaves.
Conidiosporangia produced during early phase of the growing season causes secondary infection in the host and these are blown away by wind or any other agency, land on the host surface and germinate to form zoospores.
When the conditions become unfavourable or during the later phase of the growing season, the fungus begins sexual reproduction producing oospores, which are thick-walled, and can withstand the unfavourable conditions.
The pathogen can be spread by many different methods including infected produce and leaves, infected crop debris, rain or irrigation splash, wind and soil, infected seedlings and/or contaminated seed.
INFECTION AND SYMPTOMS
The disease can infect a crop, both locally and/or systemically and symptoms vary depending on infection type. It mainly infects plant parts that contain chlorophyll (i.e., the green parts), however systemic infections can affect any part.
Symptoms of localised infections are white ‘blisters’, which in the early stages are seen on the underside of leaves. These blister-like masses cannot be scraped off without damaging the leaf.
As infection progress, circular areas of light green to yellow discolouration appear on the upper leaf surface, corresponding to white blisters on the underside of the leaves. The blisters contain white powdery spores which are spread by wind when released.
The stem and leaves of attacked crops can become twisted and deformed.
White rust disease also infects the floral parts as well. Infected broccoli and cauliflower produce grossly deformed heads and their seeds are usually sterile.
With systemic infections, the disease grows throughout the plant’s tissue, and these may be quite difficult to control. Abnormal growth, distortion of affected plants or the formation of galls indicate systemic infection.
The galls and distorted plant parts contain another type of spore which can be spread in crop debris and on seed. In seedlings, affected plants may look ‘leggy’ and taller than non-affected seedlings due to the abnormal growth.
When the systemic infection has taken early, the growth of the entire crop is checked, stunted and only small leaves may be formed.
The stem and the axis of the inflorescence may get twisted appearing in a zigzag sequence and normal dormant buds are stimulated which grow into lateral shoots.
The conditions required for white blister infection are temperature range of 13 to 25°C and leaf wetness for a minimum of two to three hours (from dew, fog, rain or irrigation).
Symptoms develop 6 to 21 days after infection. Severely infected plants wilt and die.
Registered fungicides should be used in conjunction with cultural management practices as part of an integrated pest management program. Brassica producers should not depend on a single method to manage the disease.
A fungicide resistance management strategy should also be followed because Albugo candida, which causes the white blister disease, can rapidly develop resistance to fungicides.
The following fungicides are recommended for use in management of the disease.
- ABSOLUTE® 375SC 10ml/20l
- COLONIZER® 440WP 50g/20l
- EXEMPO CURVE® 250SC 15ml/20ml
- DUCASSE® 250EC 20ml/20l
- DOMAIN® 250EC 10ml/20l
- JUPITER® 125SC 15ml/20l
- MILLIONAIRE® 690WDG 40g/20l
- MILESTONE® 250EC 10ml/20l
- TRINITY GOLD® 452WP 50g/20l
- BRADLEY® 500SC 10ml/20l
- Prevent the movement of soil and organic matter into the nursery production area.
- Producers rearing their own seedlings should follow the information for production nurseries, ensuring seedling production areas are not located close to cropping areas.
- Regularly inspect the crops for symptoms of the disease.
- Remove volunteer brassica crop plants and weeds, particularly around crops and in non-host crops.
- Thoroughly clean machinery and equipment leaving the farm to prevent build-up of soil and organic matter.
- Increase ventilation between plants by aligning planting rows in the direction of the prevailing wind and/or increasing the space between plants and planting rows.
- Plant varieties that are less susceptible to the disease.
- Ensure adequate crop nutrition, which helps to reduce the susceptibility of brassicas to white blister.
- Practise rotation with non-host plants to reduce the likelihood of disease carryover.
- Reduce leaf moisture by avoiding sprinkler irrigation
- Always mix the fungicide with INTEGRA 3ml/20l, which is a sticker, spreader and penetrant that improves efficacy of the chemical.
- Repeat sprays should be done for effective control.
- Timely management of the disease is very important.