Symptoms
Ratoon stunting disease (RSD) has no unique external symptoms. Affected crops are lower yielding that healthy crops, but the extent is dependent on variety and weather conditions, particularly the amount of moisture stress experienced during the growing season. Dry weather leads to higher yield losses.

The disease tends to give an "up and down" appearance to diseased fields, a result of variation in stunting in diseased stools. The only symptoms which can be associated with the disease are a discolouration of vascular bundles in the nodal region. In mature cane sliced longitudinally with a sharp knife, discoloured vascular bundles (yellow, orange, pink, red, reddish-brown) are seen in the shape of the dots and commas, on the lowerside of the node. The length of this discolouration varies from 1-3mm. Some varieties show excellent symptoms, others show very few. The slicing of immature stalks may reveal a pink blush through the nodal region.

Cause
RSD is caused by a small bacterium, Clavibacter xyli f.sp.xyli. The bacterium lives in the vascular tissue of the cane plant, and is highly infectious . Transmission is principally through diseased planting material and by cutting implements (planters, cane knives, chain saws, harvesters etc).

History of Incidence
RSD was first discovered (worldwide) in the Mackay district in central Queensland in the summer of 1944. It was later shown to be present throughout Queensland, and indeed the whole world!. Due to difficulties in diagnosis, it has been difficult to gauge the percentage of fields.

Distribution
RSD occurs in all cane growing districts in Australia.

Assessment of Severity
Assay
The only test for RSD used to be the slicing of mature stalks to assess internal stalk symptoms. As some varieties do not show reliable internal symptoms, this method has been discarded in favour of direct observation of the bacteria in xylem extracts using phase contrast microscopy (PCM), or a serological technique incorporating specific antibodies and linked enzyme colour changes (EIA).

PCM has been adopted widely by CPPBs and provides an accurate assay for small numbers of plant sources. EIA, undertaken in special laboratories (currently at BSES Tully and Mackay, the latter in co-operation with the Mackay CPPB), is excellent for the assay of large numbers of samples. RSD surveys and quality assurance inspections of approved seed plots are ideally undertaken using the EIA assay.

Timing and inspections
Assays for RSD rely on the build up of bacterial numbers within the vascular tissue. Assays should be conducted on maturing cane after April, as the bacterium may be undetectable earlier.

Assessing severity
RSD severity is not normally recorded since the relationship between bacterial numbers in a stalk and yield effects in individual varieties is unknown. Additionally, the logistics of assessing severity in whole blocks of cane precludes any assessment in the commercial situation. Data recording therefore normally identifies whole blocks with RSD

RSD severity is not normally recorded since the relationship between bacterial numbers in a stalk and yield effects in individual varieties is unknown. Additionally, the logistics of assessing severity in whole blocks of cane precludes any assessment in the commercial situation. Data recording therefore normally identifies whole blocks with RSD

 

Internal symptoms of RSD (diseased on left, healthy on right)	Comparison of healthy and RSD infected cane (diseased on left, healthy on right)