A fungal disease has devastated commercial feijoa crops in Northland but a scientist says backyard growers shouldn't panic.

The anthracnose fungal disease causes the fruit to fall off trees while still small and has wiped out entire crops this season.

While a warm summer meant many feijoa growers around the country had been expecting a high quality crop, none of the fruit grown on Peter Jack's Kerikeri property had made it to sale.

"Mostly with these newer varieties [of feijoa] you get early fruit drop way back in the beginning of January where the fruit is only 25mm long and it falls off," he told Radio New Zealand.

"If they're not, they certainly should be. With an insect you can kill it but with a fungus it's way more difficult."

Plant and Food Research scientist Pia Rheinlander said if it spread south, the fungus could have a massive impact on the feijoa industry.

However, it was not carried by wind like myrtle rust, another fungal disease which affected trees including pōhutukawa, mānuka and rātā.

"This is spread by rain splash, not the wind," she said.

"When affected fruit is left on the ground, the spores multiply and when rain hits it, they spread."

The current fungicide spray programme had no effect on the anthracnose fungus so growers needed to remove any affected fruit from their orchards and prune back affected branches.

Rheinlander is involved in a three-year project to develop a control programme to protect the industry from the disease.

She said it had not yet been seen outside Kerikeri or in backyard trees and its slow spread meant scientists had time to find feijoa varieties with higher resistance.

"There are a lot of different cultivars and some of them are more resistant than others," she said.

"Finding them takes time but the research is being done to ensure there are resistant varieties for the backyard."

The fungus started with a small purple-black spot which spread quickly to cover the fruit within a few days.  Although affected fruit was safe to eat, it had a rotten taste.

The NZ Feijoa Growers Association has about 140 members, producing about 1200 tonnes of fruit from 240 hectares. Most of the estimated $4 million crop is sold locally, with about $500,000 worth exported to Australia.

Earlier this year, the Ministry for Primary Industries announced it would put $356,000 towards the control programme project.

It is the second feijoa-focused project to receive funding through MPI's Sustainable Farming Fund in recent years with another focusing on the guava moth.

First reported in Northland 20 years ago and now found in Auckland, Waikato and the Coromandel, the moth lays its eggs on a feijoa and larvae then burrow into the flesh, spoiling the fruit.