White Blister
Description
Infection occurs either locally and or systemically and the symptoms expressed vary depending on the infection type. Generally, WB affects parts of the plant containing chlorophyll, however, systemic infections can affect the whole plant. Localised infections cause white blisters on the underside of the leaf during early stages of infection, which develop into corresponding round areas of light yellow to green leaf discolouration on the upper surface.
Systemic infections are difficult to control and symptoms include; abnormal growth, plant distortion and production of galls. Sexual spores (oospores) from systemic infections are produced in galls and distorted plant parts, which contaminate seed and plant debris. Affected seedlings appear spindly and taller than unaffected seedlings. Ideal conditions for infection to occur include temperatures of 13 °C – 25 °C and leaf wetness of at least 2 hours. Once infected, symptoms appear within 6 – 21 days.
Spread of WB can occur from infected plants, including brassica weeds, plant debris, diseased seed or infected seedlings. Spores (zoospores) released from contaminated soil, plant debris and seed are spread by rain splash onto plants. Zoospores germinate and infect leaf tissue forming blisters (white pustules) 10-14 days after infection. Blisters contain vegetative spores (sporangia) which then spread by wind, rain and insects. Volunteer brassica plants, including brassica weeds can act as hosts and if not controlled will contribute to the spread of WB.
Control
Management of WB can be achieved through a combination of cultural and chemical tactics. Practising good hygiene is crucial, therefore brassica producers and seedling production nurseries should thoroughly clean equipment and machinery. Crop residues should be destroyed by ploughing or discing in all crop debris after harvest and volunteer plants and other host weeds should be controlled. Plant varieties which are less susceptible to WB should be grown, particularly if the risk of WB is high. Consider planting seedlings on wide rows and in the direction of prevailing winds, to help increase ventilation between plants. Ensure plants have adequate nutrition in particular phosphorus and potassium, however, be cautious applying too much nitrogen as this can increase plant susceptibility to WB. To reduce the likelihood of infection, ensure leaves are not wet for too long and consider a short heavy watering rather than a long light watering.
Fungicides play an important role in the management of WB. Infinito® provides excellent control over WB and is best applied as part of a protectant program. Apply Infinito when favourable conditions for disease development occur and or at the first signs of infection. Infinito is registered for the control of WB in brassica vegetables and leafy brassica vegetables and should only be applied up to three times per crop.
References
Agriculture Victoria, (2021), ‘White blister on broccoli’, Agriculture Victoria, White blister on broccoli | Vegetable diseases | Plant diseases | Biosecurity | Agriculture VictoriaDepartment of Primary Industries and Regional Development (DPIRD), (2017), ‘White blister disease in vegetables brassica crops’, DPIRD, White blister disease in vegetable brassica crops | Agriculture and Food
Lancaster, R. (2003), ‘White blister control in vegetable brassica crops’, Department of Primary Industries and Regional Development (DPIRD), Farmnote 112, Farmnote 112 : White blister control in vegetable brassica crops [WA AGRIC] (ausveg.com.au)