Plant Pathogens

1. Plant Pathogens Lecture 13

2. Pathogenic bacteria • Pseudomonas syringae causes leaf spots • Pseudomonas fluorescens causes soft rot in potatoes • Erwinia carotova causes soft rot • Streptomyces scabies cases potato scab • Agrobacterium tumefaciens causes crown gall disease

3. Soft Rot • Caused by several different microbes

4. Stewart's Wilt • Caused by Pantoea stewartii • Its primary vector is the corn flea beetle • The bacterium lives in the gut of the beetle during the off season and is spread between plants when the beetle feeds on the seedlings

5. Crown Gall Disease The disease causes the formation of tumor-like swellings called galls that can generally be found on the crown of the plant just above the soil ay reduce the value of a plant in a nursery Crown gall disease does not usually seriously harm older plants; however, it may reduce the value of a plant in a nursery

6. Crown Gall Disease • Cause by Agrobacterium tumefaciens • Gram-negative, non-sporeforming, rod-shaped bacterium

7. Crown Gall Disease • The disease causes the formation of tumor-like swellings called galls that can generally be found on the crown of the plant just above the soil

8. Ti plasmid • Most of the disease causing genes are carried on a tumor inducing (Ti) plasmid • It causes "plant cancer" or what are known as galls. They are little tumors in which the bacteria can grow and live in the plant. • The bacterium gets into the plant via some kind of wound, ie. a scratch. • It injects its plasmid into a plant cell and the plasmid inserts its DNA into the plant's DNA. • The DNA then directs the plant to make a hollow tumor where the bacteria can live.

10. Why do we care about Ti plasmids? • Scientists have taken advantage of the plasmids ability to insert foreign DNA into a plant. They take the genes out of the plasmid that cause galls, and insert genes of interest, • ie. genes for pest resistance and let the plasmid carry those genes into the plant. • The plant will start making the product you have engineered it to make.

11. Green fluorescent protein-glowing Tobacco

12. How do we control/prevent bacterial disease in agricultural products?

13. Control and Prevention • Biocontrol • Minimize bacterial contamination in seeds • Sanitize and disinfest seed-handling equipment • Avoid injuring seed and allow cut seed to heal before planting • Crop rotation will help reduce the disease • Avoid planting in overly wet or dry soil

14. Control and Prevention • Avoid over-irrigation. • More frequent irrigation for a shorter time is less favorable for disease. • Do not harvest until plants are fully mature • Avoid injuries to plant • Provide adequate air flow to promote drying, particularly when product first enter storage. • Avoid packing and storing wet product

15. Biological Control • Agrobacterium tumefaciens strain K84 • Produces bacteriocins that inhibit closely-related tumor-producing strains. • The bacteriocin, agrosin, is a fraudulent adenine nucleotide that inhibits DNA synthesis • It is taken up by strains of A. tumefaciens that synthesize opines • Virulent bacterial mutant strains that do not take up the opine are not sensitive to agrosin

16. Summary • Controlling diseases in crops will be achieved through • Improved agricultural practices • Genetic engineering of plants to resist pests • New crops will replace conventional crops in many regions of the world • Biofuels vs. food crops in places like Montana • Global climate change will force changes in agriculture