Australian police are investigating a mysterious mass poisoning of seven million tomato plants and other crops which cost tens of millions of dollars in damage. <br/><br/>Detectives were investigating whether vandals or a competitor with a grudge had put herbicide in sprinklers at a major nursery near the northeastern city of Cairns, wiping out 16 million tonnes of produce, mostly tomatoes. <br/><br/>The damaged crops also included capsicums, pumpkins, melons, aubergines and courgettes due to be harvested and transported across Australia and to New Zealand and Vanuatu in the coming months. <br/><br/>Prices are expected to double or triple as a result, and growers' profits will suffer a major hit. Produce farmers have condemned the crime as an act of industrial sabotage. <br/><br/>It is the fourth and most dramatic incident of crop sabotage in the area in the past decade, and police said they were contemplating a reward for information.