Sharl Mintoff1, Samantha Cullen1, Chris Kelly1, Maxine Piggott1 and Jeff Daniells2
1Northern Territory Department of Industry, Tourism and Trade, Darwin, NT
2Queensland Department of Agriculture and Fisheries, South Johnstone, QLD
Main trial results (plant and first ratoon)
Disease assessments were carried out fortnightly once external symptoms became apparent in a susceptible variety. Assessments included noting the appearance of external disease symptoms and internal symptoms at plant death or harvest. Disease performances of a particular variety were given a score and grouped into one of the following categories:
Highly resistant (HR) – No disease symptoms were observed within the crop cycle and may not show symptoms under high inoculum pressures.
Resistant (R) – Plants normally show no signs of infection in the presence of the pathogen. However, under high inoculum pressures low amounts of symptoms or losses may occur.
Intermediate (I) – Plants which can withstand some infection and suffer low losses under natural infestation conditions, with most completing their crop cycle. However, its susceptibility or resistance can be highly dependent on the inoculum pressure already present. With the appropriate crop management or environment to lower the inoculum levels, these should be commercially viable.
Susceptible (S) – More than 50% of plants show symptoms and/or killed due to pathogen infection.
Very susceptible (VS) – Majority of plants (more than 70%) showed severe symptoms, most of which died due to TR4.
Interestingly there appeared to be a reduction in disease severity observed across the first ratoon crop compared to the plant crop, with dramatic shifts occurring for a couple of varieties (Table 1).
CIRAD 03 and CIRAD 04 retained their resistance seen in the previous crop cycle continuing to be classified as highly resistant into the first ratoon. Dwarf French Plantain moved up into this category with no disease symptoms noted in the ratoon crop.
Varieties Asia Pacific No. 1 and CIRAD 05 both displayed low incidences of TR4 infection in the first ratoon moving them down into the resistant category. GCTCV 105 and GCTCV 217 (Figure 1) both moved up into the resistant category after displaying a slight reduction in disease development in the ratoon crop. The resistant reference Goldfinger remained in this category.
Asia Pacific No. 3 moved down into this category due to an increase in plants infected with TR4 in the ratoon crop cycle. CIRAD 06 and High Noon, showed a dramatic reduction in the number of affected plants in the ratoon crop moving them up from very susceptible (in the plant crop) into the intermediate category.
Susceptible and very susceptible
Varieties that were deemed susceptible within the first ratoon crop cycle included Hom Thong Mokho, Pisang Ceylan, PKZ and the CJ19 Selection. The intermediate reference variety Formosana displayed an increase in disease severity in the first ratoon crop and as a result was moved down into the susceptible category. The susceptibility of the very susceptible reference variety Williams did not change.
Table 1 Resistance rating of trial plants in plant crop and first ratoon
HR = highly resistant, R = resistant, I = intermediate, S = susceptible, VS = very susceptible
Figure 1 Representative resistance for certain Cavendish varieties exposed to TR4 in the first ratoon. Williams displayed prominent disease symptoms with leaf yellowing, necrosis and eventual death. The majority of plants for the resistant varieties GCTCV 105 and 217 displayed no symptoms during first ratoon. Photos courtesy of Sharl Mintoff.