FEATURE root knot nematode

Carnarvon is a key horticultural district in WA, with approximately 170 horticultural properties covering 2,000 hectares. The district produces about 22,000 tonnes of vegetables annually, valued at over $69 million.

Words Truyen Vo, Sean Kelly, Carla Wilkinson and Sarah Collins, Research Scientists, DPIRD

WHILE plant-parasitic nematodes have been identified since the 1950s, primarily in banana crops, there is limited scientific information on their impact on vegetable crops in the region. Root knot nematodes (RKN) are believed to have the greatest impact on fruiting vegetables in Carnarvon. Traditionally, nematicides and broad-spectrum soil fumigants have been used to control nematodes, but their results are inconsistent. The survey aims to better understand the extent of nematode damage to Carnarvon fruiting vegetable crops, and growers’ management strategies and chemical control methods.

Research aims

• Understand nematode genus and their incidence levels in Carnarvon tomato and capsicum crops.

• Evaluate plant damage caused by nematodes and the potential economic impact.

• Assess the effectiveness of current management practices.

• Identify potential research that is required to better understand nematode damage in Carnarvon, and

• Develop and demonstrate effective management practices for growers.

Assessing root knot damage on a tomato plants root system.

Research methods

• Collect information from growers about their knowledge of nematodes, soil preparation, crop history and any issues with nematodes.

• Take soil samples from vegetable properties at depths of 0–15 and 15–30 cm at the start and end of the cropping season.

• Extract, identify, and quantify nematodes from these samples.

• Rate nematode damage on plant roots before harvest.

Results and discussion

In the 2024 cropping season (March to November), 14 sites were monitored in Carnarvon, including 4 capsicum plots and 10 tomato plots located across commercial farms in the area. Table 1 provides a summary of the survey results.

Identifying plant parasitic nematodes and calculating nematode population density

The survey found 6 plant parasitic nematode genus on fruiting vegetable crops in Carnarvon. They were Meloidogyne, Helicotylenchus, Tylenchorhynchus, Paratrichodorus, Criconemoide and Paratylenchus. Meloidogyne, also known as RKN, which causes root galls, was the most common genus found.

The RKN population density at the start of season after planting was relatively low. Five out of the 14 trial sites were infested with RKN counts ranging from 4 to 36 RKN per 200g of soil sample. The remaining 8 sites had no RKN detected (see rows 5 and 6 in Table 1). When present, the RKN were patchy, with nematodes found in 1 to 4 out of 16 soil samples at each site.

At the end of season, there was a dramatic increase in RKN presence and population density at some sites (rows 7 and 8 in Table 1). RKN were detected in 10 of 14 trial sites with the highest average population density reaching 10 251 RKN per 200g of soil.

The egg-to-egg life cycle of RKN is completed within 3–5 weeks at 25–30ºC and mature RKN females lay 150–1,500 eggs. This explains the rapid increase and spread of RKN in the soil from the start to the end of the season.

Root knot nematode damage to the root systems was assessed for the percentage of roots that were affected by galling.

At harvest, root galls were recorded in 7 out of 12 sites (two sites were not rated). The average root gall ratings for each property ranged from 0 to 6, which corresponds to 0 to 60% of the root system affected. Further research is needed to determine the relationship between root damage and crop performance.

Grower knowledge and practices

Some of the growers participating in the study indicated that they were concerned about root galling and believed that it was limiting crop growth. However, knowledge of RKN and appropriate practices for managing RKN was limited. None of growers involved had previously tested their soil for RKN, and half of the growers (Growers 2, 3, 4, 5, 6 and 11) were unfamiliar with the cause of root knot damage.

The application of nematicides and broad-spectrum soil fumigants to “cure the root gall damage” and other soil borne pests were used by growers as a precautionary practice without knowing whether the pests are present in their soil. Many growers observed inconsistent results from chemical application and were unsure on how to improve the situation.

Conversations with growers on whether they believed that the level of nematodes in their monitored crops affected yield may provide some insight into damage thresholds. Grower 8, whose crop root damage was rated 2.2–2.8 (Table 1) was satisfied with his cherry tomato crop yield and did not believe the plants were greatly affected, whilst Growers 2 and 5, with root galls ratings 1.2–5.6 and 6.1–6.7 respectively, observed a significant reduction in their harvest.

Growers 1 and 9 used plastic mulched bed solarisation at the start of the season for 45–60 days in combination with soil chemical application. No RKN was detected in the soil sampling and no root galls were detected on these farms. The use of plastic mulched bed solarisation to control RKN on both capsicums and tomatoes should be investigated further.

On some farms control of RKN was poor despite the application of soil fumigants and nematicides.

This survey provides information on the extent of RKN damage to tomato and capsicum crops in Carnarvon.

Further work is required to answer research questions and develop guidelines to assist growers effectively manage RKN.

The results of this comprehensive survey couldn’t be achieved without the participation and contributions of 12 tomato and capsicum growers in Carnarvon. The survey team are preparing grower reports for the participants.