VG03003 Lettuce – Varnish Spot

Varnish spot, a disease affecting lettuce, is caused by the bacteria Pseudomonas cichorii, and results in brown lesions around the midrib of lettuce.

The discolouration often appears on concealed inner leaves, so lettuce with varnish spot can be transported to markets without the grower having any knowledge of the disease’s presence.

This project looked at the awareness of this disease
across the lettuce industry in Australia through a survey and industry contact.

The main key findings of the project were;

• The disease was reported in all states of Australia.
• Growers surveyed were able to recognise the disease and a large proportion had the disease present on there properties most only occasionally.
• Some growers reported that they only had minor losses due to this disease; others had whole blocks of lettuce rendered unmarketable.
• Losses ranged from 0-20% in crop figures or up to $200,000 in monetary figures.
• Growers generally thought that this disease was mainly seen in spring.

This report covers the activities undertaken during the period of the project from July 2003 till June 2005.

Summary :

Varnish spot is a bacterial disease that causes brown lesions around the midrib on lettuce (Lactuca sativa L.) leaves quite commonly under outside leaves which show no symptoms.

Varnish spot of lettuce is caused by Pseudomonas cichorii. The disease has been found in other countries and is common in Australia.

This project was established to find the awareness and seriousness of this disease across the lettuce growing regions of Australia.

Growers and others in the industry were visited, met with at industry information nights and meetings or called by telephone to obtain information on varnish spot.

In response it was found that the majority of growers had seen the disease and were able to distinguish it from other bacterial rots such as those caused by Erwinia.

All states had growers that were affected by varnish spot. It was previously thought that only some states had this disease.

It appears that varnish spot can affect a lettuce sporadically or totally wipe out a planting.

One hydroponic grower had also recorded heavy losses as a result of infection by Pseudomonas cichorii.

Processors reported that it was an issue for lettuce processed for hearts, with one processor recording a 3% loss from this disease.

Seed companies agreed that most states had the disease with one company having problems with varnish spot on some varieties in 2005.

Previous research on this disease shows that it can infect through stomata and epidermal hairs.

The bacteria can survive in lettuce residue so crop rotation has been suggested as a control option.

But reports through this project have indicated that varnish spot has occurred on blocks that have not had lettuce for up to three years.

Other sources of inoculum include seeds, transplant and insects. Some growers in the survey considered water sources as a possible source of bacteria; this has been supported with overseas information.

Other hosts of P. cichorii include chicory cabbage, cauliflower, celery tobacco and endive. Numerous weed hosts could also exist.

Recommendations :

• Water used for seedlings should be tested for varnish spot bacteria.
• Transplants should be inspected for signs of disease and infected transplants destroyed.
• Irrigation should be carried out to minimize leaf wetness duration and especially reduced to a minimum within three weeks of harvesting.
• Fields are rotated for 4-5 years between lettuce crops or other hosts of the pathogen causing varnish spot.
• Hosts should not be planted in adjacent fields.
• Plant less susceptible lettuce varieties.
• Lettuce should be planted in fields with well-drained soil and good air movement to promote rapid drying.

Acknowledgements :

Thanks are expressed to staff at the National Vegetable Industry Centre, Yanco for the assistance with this project.

Growers and others in the industry for the excellent feedback and cooperation.

Also to Ric Cother and Dorothy Noble (NSW DPI-Orange Agricultural Institute) for assistance with bacterial identifications.

This project has been facilitated by NSW Department of Primary Industries and Horticulture Australia Limited (HAL) in partnership with AUSVEG and has been funded by the National Vegetable Research and Development Levy.

The Australian Government provides matched funding for all HAL’s R&D activities.