[pj46]
TITLE
Effects of soil-borne Rhizoctonia solani Kuhn on yield and quality of ten potato cultivars

INTRO
Rhizoctonia solani Kuhn is a serious pathogen of potato ( Solanum tuberosum L. ) , causing stem canker and stolon pruning.
Infections can occur from both tuber- and soil-borne inoculum.
Scholte ( 1987 ) showed that he infection through soil-bourn inoculum depended on the frequency with which potatoes had been grown; preceding crops other than potato had little effect, a finding confirmed by Specht & Leach ( 1987 ).

The effects of stem canker and stolon pruning caused by R. solani, on growth, yield or quality have been investigated by Cother & Cullis ( 1985 ) , Roth ( 1985 ) , and Hide et al. ( 1985 ) .
There was stem canker and stolon pruning in their experiments, but whereas Hide et al. artificially infected their seeds tubers, in the other two studies infection could have been caused by soil- and/or tuber-borne inoculum.

This paper describes experiments designed to elucidate the contribution of R. solani to the yield depression of potato in short rotations.
The effect of soil-borne inoculum on stem and stolon attack in relation to yield and quality of the potato were investigated.
An experiment with long and short rotations supplied excellent inoculum, but had only one cultivar.
The effects of cultivar were studied in two experiments with severe levels of soil infestation.

METHOD
In 1983, 1984 and 1985, 18 plants of the cv.
Element were harvested individually from each of 32 potato plots 74 and 117 days after planting in a rotation experiment described by Scholte ( 1987: Experiment 1 ) .
The levels of R. solani infection, numbers of stem and tubers, and fresh yield, dry matter yield and dry matter content of tops and tubers were measured for each of 3456 plants.

In 1986, cv. Mirka was included in the experiment by dividing each plot into two: 83 days after planting, 18 plants of each cultivar were harvested per plot and infection by R. solani assessed.

This experiment was laid out in 1984 on a field of light sandy soil, containing 2.2% organic matter and with a pH-KCl of 5.2, where potatoes ( 1982 ) and maize ( 1983 ) had previously been grown.

In November 1983, 30 000 l ha-1 cattle slurry was applied to the field and then ploughed early in March 1984.
Before planting, N, P and K fertilizers at 201, 22 and 83 kg ha-1, respectively, were incorporated with a cultivator.

Two very early maturing cultivars, Eersteling and Ostara, and two very late maturing cultivars, Multa and Alpha, were compared at two stem densities, 12 and 24 stems per m2, achieved by planting one or two seed tubers per hill.

The experiment was a randomized complete block design with four replicates.
The plots were 9.0 x 4.5 = 40.5 m2, of which 7.5 x 3.3 = 24.75 m2 was harvested.
Seed tubers were planted on 8 April at 75 x 30 cm.
Weeds, aphids and late blight were controlled.

Two weeks before the expected maturing date, 110 plants per plot were individually harvested.
Rhizoctonia infection, stem number, and fresh and dry matter yield of tubers were recorded for each of 3520 plants.

This experiment was laid out in 1985 on light sandy soil ( 2.2% organic matter, pH-KCl 5.2 ) in a field where maize ( 1983 ) and potatoes ( 1984 ) had been grown.

To encourage R. solani, the field was treated with aldicarb ( Temik 10G, Union Carbide Benelux, Maarsen, 10% a.i., 30 kg ha-1 broadcast ) one day before planting.

The early maturing cultivars Eersteling, Saskia and Vindika and the late maturing cultivars Alpha, Amigo and Baraka were compared for their sensitivity to R. solani.

Experimental design, plot size, plant spacing and other cultural practices were as for Experiment 2.
The planting date was 10 April.
Harvesting and recording of 2640 plants was also done as in Experiment 2 but tuber quality was additionally assessed.

In all experiments R. solani attack was classified as

Yields in the group with class 1 attack were found not to have suffered ( Experiment 1 ) and infections in this group could not irrefutably be attributed to R. solani, therefore classes 0 and 1 were combined in Experiment 2 and 3.
As there were few plants of class 4 in Experiments 1 and 2, class 4 was combined with class 3.

Before planting, seed tubers ( 32 - 35 mm ) visibly free from black scurf were immersed in a solution of validamycine ( Solacol, AAgrunol, Haren, 30 g I-1 a.i., 3 % solution ) to control superficial R. solani mycelium.

For Experiments 1 and 2, seed tubers were not pre-sprouted.
For Experiment 3, tubers were pre-sprouted for two weeks in light and short, dark sprouts developed on the tubers.
Planting was done by hand.

RESULTS
These are the results, we are not using them.

DISCUSSION
Because the seed tubers planted in the experiments were free from black scurf and had been disinfected, only soil-borne inoculum of R. solani could be held responsible for infections.

Severe attacks reduced the yield and quality of tubers but a slight or moderate attack had little or no effect.
The fresh tuber yields of severely and very severely attacked plants were reduced by ca. 16 and 21%, respectively, and dry matter tuber yields were reduced by ca. 20 and 29%, respectively.
Marketable yield was reduced even more because the proportion of tubers > 35 mm decreased and the proportion of misshapen tubers greatly increased.

Roth ( 1985 ) and Cother & Cullis ( 1985 ) also used an individual plant sampling procedure to study the effects of R. solani infections on yield.
However, because they did not disinfect their seed tubers, their R. solani stem and stolon infections could have been caused by soil-bourne and/or tuber-bourne inoculum.
In severely attacked plants, Roth found fresh tuber yields reduced by 13 -24 %, depending on cultivar.
Cother & Cullis found that when all stolons of a plant were pruned, fresh tuber yield was reduced by 24 %.

Hide et al. ( 1985b ) inoculated seed tubers in the ridge at planting and found yields reduced by 4 % compared to their controls.
Chand & Logan ( 1982 ) found, averaged over ten cultivars, a fresh yield reduction of 18% in plots artificially inoculated with R. solani compared with uninoculated plots.

Other workers have applied specific fungicides to the soil to study the effects of soil-bourne R.solani on yield.
Cother ( 1983 ) and Davis ( 1978 ) used pesticides based on quinozene ( PCNB ), but this is not a good standard because, at the recommended dosages, it is phytotoxic to potato, lowering yields ( Van Emden, 1958 ) .
In two experiments on sandy soils, Moulder & Roosjen ( 1982 ) planted disinfected seed tubers in plots untreated and treated with tolclofos-methyl, furmecyclox and pencycuran.
At the recommended dosages, dry matter yields of tubers were 4 - 13 % higher on treated than on untreated plots.

The success of soil-applied fungicides depends on the inoculum level of R. solani.
In plots on sandy soil continuously cropped with potato Scholte ( 1987 ) found 30% severely attacked plants, averaged over four years.
The yield reduction of such a crop can be calculated with formulae derived from the results of the experiments discussed in this report.
The relationships between the percentage of severe ( S ) and very severe ( V ) attacks of stem stolons caused by R. solani and the related total fresh ( F ) and dry matter ( D ) tuber yield can be expressed as :

For crops with 30% severely attacked plants, the fresh and dry matter tuber yields are decreased by about 5 % and 6 %, respectively.
As these values are close to the limit for significance at P < 0.05, it is difficult to prove that the beneficial effects of soil applied fungicides are statistically significant.

The negative effect of an R. solani infection on tuber yield tends to decrease towards the end of the growing season: for example 36 % lower yield on day 74 and 20 % on day 117 for severely attacked plants ( Table 1 ) .
A similar effect was found by Griffith ( 1984 ) and Hide et al. ( 1985b ) .

Young sprouts are very susceptible to R. solani infection and early infection can prune off a stem as occurs especially with tuber-borne rather than soil-borne inoculum ( Frank, 1987; Van Emden et al., 1966 ) .
This phenomenon may explain why the effects of attack on the number of main stems were small in Experiment 1 and 2.

Van Emden ( 1965 ) concluded that when sprouts were exposed to light after emergence they became resistant to R. solani but this does not always seem to be true.
Scholte ( 1987 ) , Roth ( 1985 ) and Hide et al. ( 1985a ) found a marked increase in stem infection after emergence.
Susceptibility seems to decrease gradually with increasing age of stems, probably due to a change in the structure and composition of the outer-most cell layers.
Similarly, sprouts formed in the light are not attacked by R. solani but when seed tubers that have been pre-sprouted under light are planted deeply, the part of the sprout which forms between the original sprout tip and the soil surface is again susceptible to R. solani ( Roosjen & Mulder, 1982 ) .

Cother & Cullis ( 1985 ) found no significant relationship between stem canker and tuber yield an concluded that only stolon pruning results in lower yields.
This seems debatable; the effect of stolon attack may be more important for yield than stem attack, but stem attack also affects growth of the plant.
A severe stem attack on a young plant results in diminished growth ( Table 1, day 74 ) .
Clearly, the mean weight of the stems is lower for severely attacked than for healthy plants; this is true in young plants even for those moderately attacked.
However, a severe stem attack is often associated with severe stolon attack, a relationship also found by Weinhold et al. ( 1982 ) .
However, late stolon attack may occur without a stem infection.

Initially, stem canker and stolon pruning have a large depressive effect on total plant weight.
However, tuber yield is reduced more than haulm yield, and tuber number is also reduced.
Stolon infection delays tuber initiation and , as a result, tuber yield is slower to develop.
Stem canker and stolon pruning change the distribution pattern of dry matter within the plant.
Translocation of carbohydrates from the haulm to underground plant parts is hampered, dry matter accumulates in the haulm so that in attacked plants the haulm dry matter content is higher and that in the tubers is lower compared with healthy plants.
These trends are maintained later in the growing season but the initial differences in haulm weight disappear.
Hide et al. ( 1985b ) found a lower weight for main stems, but this was compensated by a higher weight for lateral stems.
In severely attacked plants tuber number per plant was initially lower, but later in the growing season it was higher than in plants that had not been attacked.
In very severely attacked plants, the Ôlittle potatoÕ plants, only a few tubers remained on the underground stem bases; they usually grew into large tubers, whereas near the soil surface many small tubers developed.

Plants severely and very severely attacked by R. solani appear to be continuously in a state of secondary growth, resulting in deformed tubers ( elongated and knobby tubers with protruding eyes ) .
The second-growth phenomena are probably the result of fluctuating hormone levels in the plant.
Tubers situated near the soil surface, sometimes partly exposed, are also exposed to higher temperatures and this also promotes second growth.
the dormancy of these tubers is low ( Gadewar et al., 1980 ) .

The effects of an R. solani attack on tuber yield and quality depend on the cultivar and perhaps, on circumstances.
For example in Experiment 2, the two late maturing cultivars were attacked more severely than the two early maturing cultivars.
The seed tubers had been planted directly from cold store and were not pre-sprouted so that the late maturing cultivars emerged much later than the early maturing ones.
The latter therefore ÔescapedÕ R. solani attack.

Of the two early maturing cultivars, Eersteling was more seriously ( though not significantly so ) attacked than Ostara, whereas of the late maturing cultivars, Multa was much more severely attacked than Alpha.
In Experiment 3, seed tubers were pre-sprouted, differences in date of emergence were small, R. solani attack was serious, and the differences in attack between cultivars were statistics significant, but were not related to the earliness of the cultivar.
Differences were also found in the number of very severely attacked ( Ôlittle potatoÕ ) plants.
Cvs Alpha and Baraka had the same number of plants the were at least severely attacked, but the proportion of Ôlittle potatoÕ plants was much higher for cv. Alpha than for cv. Baraka.
Cv. Eersteling had very many Ôlittle  potatoÕ plants.

Experiment 1 also revealed that cultivars differ in susceptibility.
The level of attack was twice as high in cv. Mirka as in cv. Element.
Difference between cultivars in susceptibility to R. solani were also found by Assenov ( 1986 ) , Bogucka ( 1983 ), Plamadeala ( 1978 ) , Frank et al. ( 1976 ) , Dowley ( 1972 ) and Hofferbert & Orth ( 1951 ) .

One conclusion from Experiment 2 and 3 is that if cultivars are to be compared for their resistance to R. solani, precautions should be taken, such as pre-sprouting the seed tubers, to ensure that the time between planting and emergence is about the same for all cultivars.

In Experiments 2 and 3 there was no evidence that cultivars differ in sensitivity to R. solani; at the same level of attack there were no significant differences in yield reductions.

Severe and very severe attacks of stems and stolons by soil-borne R. solani decreases fresh yield, dry matter yield and dry matter content of tubers, increases the proportion of deformed and small tubers, but have a negligible effect on haulm yield and stem number.

Slight and moderate attacks have very small or no effects on yield and quality.

The relative fresh ( F ) or dry matter ( D ) yield of tubers can be estimated from the percentage of severely ( S ) and very severely ( V ) attacked plants by the formulae.

Potato cultivars differ in susceptibility to stem and stolon infection by R. solani, but there is no evidence that they differ in sensitivity.